How to loop over axis plots in python - python

I am trying to create an axis plot. I was trying to loop over it as I am plotting the same variable for two different categories. Currently, I have written code two times but I am looking for a smarter way with looping, if possible. Any other suggestion will also be helpful.
zone = ['AB','CD']
plt.style.use('default')
fig,(ax0,ax1) = plt.subplots(2,1, figsize = (18,18), sharex = False)
i = 0
while i < len(zone):
if zone[i] == zone[0]:
ax0.plot(df0['datetime'], df0['pnl1'], color='k', linewidth=1, label ='PnL1')
ax0.plot(df0['datetime'], df0['pnl2'], color='m', linewidth=1, label ='PnL2')
ax00 = ax0.twinx()
ax00.bar(df0['datetime'], df0['qty'], width = 1/96, color='g', align = 'edge', alpha = 0.5, label ='Qty')
elif zone[i] == zone[1]:
ax1.plot(df0['datetime'], df0['pnl1'], color='k', linewidth=1, label ='PnL1')
ax1.plot(df0['datetime'], df0['pnl2'], color='m', linewidth=1, label ='PnL2')
ax01 = ax1.twinx()
ax01.bar(df0['datetime'], df0['hedge'], width = 1/96, color='g', align = 'edge', alpha = 0.5, label ='Qty')
i = i + 1
I want to check if something like below can be done with axis plots or not.
zone = ['AB','CD']
plt.style.use('default')
fig,(ax0,ax1) = plt.subplots(2,1, figsize = (18,18), sharex = False)
i = 0
while i < len(zone):
ax{''}.format(i).plot(df0['datetime'], df0['pnl1'], color='k', linewidth=1, label ='PnL1')
ax{''}.format(i).plot(df0['datetime'], df0['pnl2'], color='m', linewidth=1, label ='PnL2')
ax0{''}.format(i) = ax{''}.format(i).twinx()
ax0{''}.format(i).bar(df0['datetime'], df0['qty'], width = 1/96, color='g', align = 'edge', alpha = 0.5, label ='Qty')
It did not work for me. Any leads to execute axis plot with loop will be helpful.

Here are some ways:
Simply loop over the list of axes
import matplotlib.pyplot as plt
import numpy as np
fig,axes = plt.subplots(2,1)
x = np.linspace(0,5,21)
for ax in axes:
ax.plot(x,np.sin(x))
plt.show()
Works also with index:
for i in range(len(axes)):
axes[i].plot(x,np.sin(x))
For a grid of plot you can use a similar approach:
import matplotlib.pyplot as plt
import numpy as np
fig,axes = plt.subplots(2,2)
x = np.linspace(0,5,21)
for i in range(len(axes)):
for j in range(len(axes[0])):
axes[i][j].plot(x,np.sin(x))
plt.show()
If you don't like double-loops, you can flatten the array with np.ravel()
fig,axes = plt.subplots(2,2)
x = np.linspace(0,5,21)
for ax in np.ravel(axes):
ax.plot(x,np.sin(x))
plt.show()

Related

ytick descriptions in seaborn

I'm trying to make a heatmap a heatmap with extensive y axis descriptions.
I would like to know if there is anyways to have a second and a third layer on the y tick labels.
fig, ax = plt.subplots(figsize=(20,25))
sns.set(style="darkgrid")
colName = [r'A', r'B', r'C', r'D', r'E']
colTitile = 'Test'
rowName = [r'a', r'b', r'c', r'd']
rowsName = [r'Vegetables', r'Fruits', r'Meats', r'Cheese',
r'Candy', r'Other']
rowTitile = 'Groups'
heatmapdata= np.arange(100).reshape(24,5)
sns.heatmap(heatmapdata,
cmap = 'turbo',
cbar = True,
vmin=0,
vmax=100,
ax=ax,
xticklabels = colName,
yticklabels = rowName)
for x in np.arange(0,len(ax.get_yticks()),4):
ax.axhline(x, color = 'white', lw=2)
Is there any way to do this? Which function should I use?
Thanks!
The labels for the rows can be set up in the graph settings, but other than that, I think the annotation function is the only way to handle this. the second level group names are set using the annotation function, and the coordinate criteria are set using the axis criteria. Axis labels are added using the text function with axis criteria.
import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np
fig, ax = plt.subplots(figsize=(10,10))
sns.set(style="darkgrid")
colName = [r'A', r'B', r'C', r'D', r'E']
colTitile = 'Test'
rowName = [r'a', r'b', r'c', r'd']
rowsName = [r'Vegetables', r'Fruits', r'Meats', r'Cheese',
r'Candy', r'Other']
rowTitle = 'Groups'
heatmapdata= np.arange(120).reshape(24,5)
sns.heatmap(heatmapdata,
cmap='turbo',
cbar=True,
vmin=0,
vmax=100,
ax=ax,
xticklabels=colName,
yticklabels=np.tile(rowName, 6))
for x in np.arange(0,ax.get_ylim()[0],4):
ax.axhline(x, color = 'white', lw=2)
for idx,g in enumerate(rowsName[::-1]):
ax.annotate(g, xy=(-100, idx*90+45), xycoords='axes points', size=14)
ax.text(x=-0.3, y=0.5, s=rowTitle, ha='center', transform=ax.transAxes, rotation=90, font=dict(size=16))
plt.show()

Seaborn plot with second y axis

i wanted to know how to make a plot with two y-axis so that my plot that looks like this :
to something more like this by adding another y-axis :
i'm only using this line of code from my plot in order to get the top 10 EngineVersions from my data frame :
sns.countplot(x='EngineVersion', data=train, order=train.EngineVersion.value_counts().iloc[:10].index);
I think you are looking for something like:
import matplotlib.pyplot as plt
x = [1,2,3,4,5]
y = [1000,2000,500,8000,3000]
y1 = [1050,3000,2000,4000,6000]
fig, ax1 = plt.subplots()
ax2 = ax1.twinx()
ax1.bar(x, y)
ax2.plot(x, y1, 'o-', color="red" )
ax1.set_xlabel('X data')
ax1.set_ylabel('Counts', color='g')
ax2.set_ylabel('Detection Rates', color='b')
plt.show()
Output:
#gdubs If you want to do this with Seaborn's library, this code set up worked for me. Instead of setting the ax assignment "outside" of the plot function in matplotlib, you do it "inside" of the plot function in Seaborn, where ax is the variable that stores the plot.
import seaborn as sns # Calls in seaborn
# These lines generate the data to be plotted
x = [1,2,3,4,5]
y = [1000,2000,500,8000,3000]
y1 = [1050,3000,2000,4000,6000]
fig, ax1 = plt.subplots() # initializes figure and plots
ax2 = ax1.twinx() # applies twinx to ax2, which is the second y axis.
sns.barplot(x = x, y = y, ax = ax1, color = 'blue') # plots the first set of data, and sets it to ax1.
sns.lineplot(x = x, y = y1, marker = 'o', color = 'red', ax = ax2) # plots the second set, and sets to ax2.
# these lines add the annotations for the plot.
ax1.set_xlabel('X data')
ax1.set_ylabel('Counts', color='g')
ax2.set_ylabel('Detection Rates', color='b')
plt.show(); # shows the plot.
Output:
Seaborn output example
You could try this code to obtain a very similar image to what you originally wanted.
import seaborn as sb
from matplotlib.lines import Line2D
from matplotlib.patches import Rectangle
x = ['1.1','1.2','1.2.1','2.0','2.1(beta)']
y = [1000,2000,500,8000,3000]
y1 = [3,4,1,8,5]
g = sb.barplot(x=x, y=y, color='blue')
g2 = sb.lineplot(x=range(len(x)), y=y1, color='orange', marker='o', ax=g.axes.twinx())
g.set_xticklabels(g.get_xticklabels(), rotation=-30)
g.set_xlabel('EngineVersion')
g.set_ylabel('Counts')
g2.set_ylabel('Detections rate')
g.legend(handles=[Rectangle((0,0), 0, 0, color='blue', label='Nontouch device counts'), Line2D([], [], marker='o', color='orange', label='Detections rate for nontouch devices')], loc=(1.1,0.8))

Single row (or column) heat map in python

I can create and n by n heatmap using the following code, for example let n be 10:
random_matrix = np.random.rand(10,10)
number = 10
incrmnt = 1.0
x = list(range(1,number +1))
plt.pcolormesh(x, x, random_matrix)
plt.colorbar()
plt.xlim(1, number)
plt.xlabel('Number 1')
plt.ylim(1, number)
plt.ylabel('Number 2')
plt.tick_params(
axis = 'both',
which = 'both',
bottom = 'off',
top = 'off',
labelbottom = 'off',
right = 'off',
left = 'off',
labelleft = 'off')
I would like to add a 2 row heatmap one near each of the x and y axis, from say row1 = np.random.rand(1,10)and col1 = np.random.rand(1,10).
Here is an example image of what I would like to produce:
Thanks in advance.
You would create a subplot grid where the width- and height ratios between the subplots correspond to the number of pixels in the respective dimension. You can then add respective plots to those subplots. In the code below I used an imshow plot, because I find it more intuitive to have one pixel per item in the array (instead of one less).
In order to have the colorbar represent the colors accross the different subplots, one can use a matplotlib.colors.Normalize instance, which is provided to each of the subplots, as well as the manually created ScalarMappable for the colorbar.
import matplotlib
import matplotlib.pyplot as plt
import numpy as np
m = np.random.rand(10,10)
x = np.random.rand(1,m.shape[1])
y = np.random.rand(m.shape[0],1)
norm = matplotlib.colors.Normalize(vmin=0, vmax=1)
grid = dict(height_ratios=[1, m.shape[0]], width_ratios=[1,m.shape[0], 0.5 ])
fig, axes = plt.subplots(ncols=3, nrows=2, gridspec_kw = grid)
axes[1,1].imshow(m, aspect="auto", cmap="viridis", norm=norm)
axes[0,1].imshow(x, aspect="auto", cmap="viridis", norm=norm)
axes[1,0].imshow(y, aspect="auto", cmap="viridis", norm=norm)
axes[0,0].axis("off")
axes[0,2].axis("off")
axes[1,1].set_xlabel('Number 1')
axes[1,1].set_ylabel('Number 2')
for ax in [axes[1,1], axes[0,1], axes[1,0]]:
ax.set_xticks([]); ax.set_yticks([])
sm = matplotlib.cm.ScalarMappable(cmap="viridis", norm=norm)
sm.set_array([])
fig.colorbar(sm, cax=axes[1,2])
plt.show()

How to set a secondary y-axis in Python

I'm currently trying to change the secondary y-axis values in a matplot graph to ymin = -1 and ymax = 2. I can't find anything on how to change the values though. I am using the secondary_y = True argument in .plot(), so I am not sure if changing the secondary y-axis values is possible for this. I've included my current code for creating the plot.
df.plot()
df.plot(secondary_y = "Market")
From your example code, it seems you're using Pandas built in ploting capabilities. One option to add a second layer is by using matplotlib directly like in the example "two_scales.py".
It uses
import matplotlib.pyplot as plt
fig, ax1 = plt.subplots()
ax1.plot(df["..."])
# ...
ax2 = ax1.twinx()
ax2.plot(df["Market"])
ax2.set_ylim([0, 5])
where you can change the y-limits.
Setting ylim on plot does not appear to work in the case of secondary_y, but I was able to workaround with this:
import pandas as pd
df = pd.DataFrame({'one': range(10), 'two': range(10, 20)})
ax = df['one'].plot()
ax2 = df['two'].plot(secondary_y=True)
ax2.set_ylim(-20, 50)
fig = ax.get_figure()
fig.savefig('test.png')
This is a solution for showing as much y-axes as data columns the dataframe has
colors = ['tab:blue',
'tab:orange',
'tab:green',
'tab:red',
'tab:purple',
'tab:brown',
'tab:pink',
'tab:gray',
'tab:olive',
'tab:cyan']
#X axe and first Y axe
fig, ax1 = plt.subplots()
x_label = str( dataFrame.columns[0] )
index = dataFrame[x_label]
ax1.set_xlabel(x_label)
ax1.set_xticklabels(dataFrame[x_label], rotation=45, ha="right")
firstYLabel = str( dataFrame.columns[1] )
ax1.set_ylabel(firstYLabel, color = colors[0])
ax1.plot(index, dataFrame[firstYLabel], color = colors[0])
ax1.tick_params(axis='y', labelcolor = colors[0])
#Creates subplots with independet y-Axes
axS =[]
def newTwix(label, ax1, index, dataFrame):
print(label)
actualPos = len(axS)
axS.append(ax1.twinx())
axS[actualPos].set_ylabel(label, color = colors[actualPos%10 + 1])
axS[actualPos].plot(index, dataFrame[label], color=colors[actualPos%10 + 1])
axS[actualPos].tick_params(axis='y', labelcolor=colors[actualPos%10 + 1])
identation = 0.075 #would improve with a dynamic solution
p = 1 + identation
for i in range(2,len(dataFrame.columns)):
newTwix(str(dataFrame.columns[i]), ax1, index, dataFrame)
if (len(axS) == 1):
axS[len(axS)-1].spines.right.set_position(("axes", p))
else:
p = int((p + identation)*1000)/1000
axS[len(axS)-1].spines.right.set_position(("axes", p))
fig.tight_layout() # otherwise the right y-label is slightly clipped
plt.subplots_adjust(left=0.04, right=0.674, bottom=0.1)
mng = plt.get_current_fig_manager()
mng.full_screen_toggle()
plt.show()
multiple y-axes with independent scales

Secondary axis with twinx(): how to add to legend?

I have a plot with two y-axes, using twinx(). I also give labels to the lines, and want to show them with legend(), but I only succeed to get the labels of one axis in the legend:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(time, Swdown, '-', label = 'Swdown')
ax.plot(time, Rn, '-', label = 'Rn')
ax2 = ax.twinx()
ax2.plot(time, temp, '-r', label = 'temp')
ax.legend(loc=0)
ax.grid()
ax.set_xlabel("Time (h)")
ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
ax2.set_ylabel(r"Temperature ($^\circ$C)")
ax2.set_ylim(0, 35)
ax.set_ylim(-20,100)
plt.show()
So I only get the labels of the first axis in the legend, and not the label 'temp' of the second axis. How could I add this third label to the legend?
You can easily add a second legend by adding the line:
ax2.legend(loc=0)
You'll get this:
But if you want all labels on one legend then you should do something like this:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
time = np.arange(10)
temp = np.random.random(10)*30
Swdown = np.random.random(10)*100-10
Rn = np.random.random(10)*100-10
fig = plt.figure()
ax = fig.add_subplot(111)
lns1 = ax.plot(time, Swdown, '-', label = 'Swdown')
lns2 = ax.plot(time, Rn, '-', label = 'Rn')
ax2 = ax.twinx()
lns3 = ax2.plot(time, temp, '-r', label = 'temp')
# added these three lines
lns = lns1+lns2+lns3
labs = [l.get_label() for l in lns]
ax.legend(lns, labs, loc=0)
ax.grid()
ax.set_xlabel("Time (h)")
ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
ax2.set_ylabel(r"Temperature ($^\circ$C)")
ax2.set_ylim(0, 35)
ax.set_ylim(-20,100)
plt.show()
Which will give you this:
I'm not sure if this functionality is new, but you can also use the get_legend_handles_labels() method rather than keeping track of lines and labels yourself:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
pi = np.pi
# fake data
time = np.linspace (0, 25, 50)
temp = 50 / np.sqrt (2 * pi * 3**2) \
* np.exp (-((time - 13)**2 / (3**2))**2) + 15
Swdown = 400 / np.sqrt (2 * pi * 3**2) * np.exp (-((time - 13)**2 / (3**2))**2)
Rn = Swdown - 10
fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(time, Swdown, '-', label = 'Swdown')
ax.plot(time, Rn, '-', label = 'Rn')
ax2 = ax.twinx()
ax2.plot(time, temp, '-r', label = 'temp')
# ask matplotlib for the plotted objects and their labels
lines, labels = ax.get_legend_handles_labels()
lines2, labels2 = ax2.get_legend_handles_labels()
ax2.legend(lines + lines2, labels + labels2, loc=0)
ax.grid()
ax.set_xlabel("Time (h)")
ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
ax2.set_ylabel(r"Temperature ($^\circ$C)")
ax2.set_ylim(0, 35)
ax.set_ylim(-20,100)
plt.show()
From matplotlib version 2.1 onwards, you may use a figure legend. Instead of ax.legend(), which produces a legend with the handles from the axes ax, one can create a figure legend
fig.legend(loc="upper right")
which will gather all handles from all subplots in the figure. Since it is a figure legend, it will be placed at the corner of the figure, and the loc argument is relative to the figure.
import numpy as np
import matplotlib.pyplot as plt
x = np.linspace(0,10)
y = np.linspace(0,10)
z = np.sin(x/3)**2*98
fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(x,y, '-', label = 'Quantity 1')
ax2 = ax.twinx()
ax2.plot(x,z, '-r', label = 'Quantity 2')
fig.legend(loc="upper right")
ax.set_xlabel("x [units]")
ax.set_ylabel(r"Quantity 1")
ax2.set_ylabel(r"Quantity 2")
plt.show()
In order to place the legend back into the axes, one would supply a bbox_to_anchor and a bbox_transform. The latter would be the axes transform of the axes the legend should reside in. The former may be the coordinates of the edge defined by loc given in axes coordinates.
fig.legend(loc="upper right", bbox_to_anchor=(1,1), bbox_transform=ax.transAxes)
You can easily get what you want by adding the line in ax:
ax.plot([], [], '-r', label = 'temp')
or
ax.plot(np.nan, '-r', label = 'temp')
This would plot nothing but add a label to legend of ax.
I think this is a much easier way.
It's not necessary to track lines automatically when you have only a few lines in the second axes, as fixing by hand like above would be quite easy. Anyway, it depends on what you need.
The whole code is as below:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
time = np.arange(22.)
temp = 20*np.random.rand(22)
Swdown = 10*np.random.randn(22)+40
Rn = 40*np.random.rand(22)
fig = plt.figure()
ax = fig.add_subplot(111)
ax2 = ax.twinx()
#---------- look at below -----------
ax.plot(time, Swdown, '-', label = 'Swdown')
ax.plot(time, Rn, '-', label = 'Rn')
ax2.plot(time, temp, '-r') # The true line in ax2
ax.plot(np.nan, '-r', label = 'temp') # Make an agent in ax
ax.legend(loc=0)
#---------------done-----------------
ax.grid()
ax.set_xlabel("Time (h)")
ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
ax2.set_ylabel(r"Temperature ($^\circ$C)")
ax2.set_ylim(0, 35)
ax.set_ylim(-20,100)
plt.show()
The plot is as below:
Update: add a better version:
ax.plot(np.nan, '-r', label = 'temp')
This will do nothing while plot(0, 0) may change the axis range.
One extra example for scatter
ax.scatter([], [], s=100, label = 'temp') # Make an agent in ax
ax2.scatter(time, temp, s=10) # The true scatter in ax2
ax.legend(loc=1, framealpha=1)
Preparation
import numpy as np
from matplotlib import pyplot as plt
fig, ax1 = plt.subplots( figsize=(15,6) )
Y1, Y2 = np.random.random((2,100))
ax2 = ax1.twinx()
Content
I'm surprised it did not show up so far but the simplest way is to either collect them manually into one of the axes objs (that lie on top of each other)
l1 = ax1.plot( range(len(Y1)), Y1, label='Label 1' )
l2 = ax2.plot( range(len(Y2)), Y2, label='Label 2', color='orange' )
ax1.legend( handles=l1+l2 )
or have them collected automatically into the surrounding figure by fig.legend() and fiddle around with the the bbox_to_anchor parameter:
ax1.plot( range(len(Y1)), Y1, label='Label 1' )
ax2.plot( range(len(Y2)), Y2, label='Label 2', color='orange' )
fig.legend( bbox_to_anchor=(.97, .97) )
Finalization
fig.tight_layout()
fig.savefig('stackoverflow.png', bbox_inches='tight')
A quick hack that may suit your needs..
Take off the frame of the box and manually position the two legends next to each other. Something like this..
ax1.legend(loc = (.75,.1), frameon = False)
ax2.legend( loc = (.75, .05), frameon = False)
Where the loc tuple is left-to-right and bottom-to-top percentages that represent the location in the chart.
I found an following official matplotlib example that uses host_subplot to display multiple y-axes and all the different labels in one legend. No workaround necessary. Best solution I found so far.
http://matplotlib.org/examples/axes_grid/demo_parasite_axes2.html
from mpl_toolkits.axes_grid1 import host_subplot
import mpl_toolkits.axisartist as AA
import matplotlib.pyplot as plt
host = host_subplot(111, axes_class=AA.Axes)
plt.subplots_adjust(right=0.75)
par1 = host.twinx()
par2 = host.twinx()
offset = 60
new_fixed_axis = par2.get_grid_helper().new_fixed_axis
par2.axis["right"] = new_fixed_axis(loc="right",
axes=par2,
offset=(offset, 0))
par2.axis["right"].toggle(all=True)
host.set_xlim(0, 2)
host.set_ylim(0, 2)
host.set_xlabel("Distance")
host.set_ylabel("Density")
par1.set_ylabel("Temperature")
par2.set_ylabel("Velocity")
p1, = host.plot([0, 1, 2], [0, 1, 2], label="Density")
p2, = par1.plot([0, 1, 2], [0, 3, 2], label="Temperature")
p3, = par2.plot([0, 1, 2], [50, 30, 15], label="Velocity")
par1.set_ylim(0, 4)
par2.set_ylim(1, 65)
host.legend()
plt.draw()
plt.show()
If you are using Seaborn you can do this:
g = sns.barplot('arguments blah blah')
g2 = sns.lineplot('arguments blah blah')
h1,l1 = g.get_legend_handles_labels()
h2,l2 = g2.get_legend_handles_labels()
#Merging two legends
g.legend(h1+h2, l1+l2, title_fontsize='10')
#removes the second legend
g2.get_legend().remove()
As provided in the example from matplotlib.org, a clean way to implement a single legend from multiple axes is with plot handles:
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
fig.subplots_adjust(right=0.75)
twin1 = ax.twinx()
twin2 = ax.twinx()
# Offset the right spine of twin2. The ticks and label have already been
# placed on the right by twinx above.
twin2.spines.right.set_position(("axes", 1.2))
p1, = ax.plot([0, 1, 2], [0, 1, 2], "b-", label="Density")
p2, = twin1.plot([0, 1, 2], [0, 3, 2], "r-", label="Temperature")
p3, = twin2.plot([0, 1, 2], [50, 30, 15], "g-", label="Velocity")
ax.set_xlim(0, 2)
ax.set_ylim(0, 2)
twin1.set_ylim(0, 4)
twin2.set_ylim(1, 65)
ax.set_xlabel("Distance")
ax.set_ylabel("Density")
twin1.set_ylabel("Temperature")
twin2.set_ylabel("Velocity")
ax.yaxis.label.set_color(p1.get_color())
twin1.yaxis.label.set_color(p2.get_color())
twin2.yaxis.label.set_color(p3.get_color())
tkw = dict(size=4, width=1.5)
ax.tick_params(axis='y', colors=p1.get_color(), **tkw)
twin1.tick_params(axis='y', colors=p2.get_color(), **tkw)
twin2.tick_params(axis='y', colors=p3.get_color(), **tkw)
ax.tick_params(axis='x', **tkw)
ax.legend(handles=[p1, p2, p3])
plt.show()
Here is another way to do this:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
fig = plt.figure()
ax = fig.add_subplot(111)
pl_1, = ax.plot(time, Swdown, '-')
label_1 = 'Swdown'
pl_2, = ax.plot(time, Rn, '-')
label_2 = 'Rn'
ax2 = ax.twinx()
pl_3, = ax2.plot(time, temp, '-r')
label_3 = 'temp'
ax.legend([pl[enter image description here][1]_1, pl_2, pl_3], [label_1, label_2, label_3], loc=0)
ax.grid()
ax.set_xlabel("Time (h)")
ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
ax2.set_ylabel(r"Temperature ($^\circ$C)")
ax2.set_ylim(0, 35)
ax.set_ylim(-20,100)
plt.show()
enter image description here
The solutions proposed so far have one or two inconvenients:
Handles needs to be collected individually when plotting, e.g. lns1 = ax.plot(time, Swdown, '-', label = 'Swdown'). There is a risk of forgetting handles when updating the code.
Legend is drawn for the whole figure, not by subplot, which is likely a no-go if you have multiple subplots.
This new solution takes advantage of Axes.get_legend_handles_labels() to collect existing handles and labels for the main axis and for the twin axis.
Collecting handles and labels automatically
This numpy operation will scan all axes which share the same subplot area than ax, including ax and return merged handles and labels:
hl = np.hstack([axis.get_legend_handles_labels()
for axis in ax.figure.axes
if axis.bbox.bounds == ax.bbox.bounds])
It can be used to feed legend() arguments this way:
import numpy as np
import matplotlib.pyplot as plt
t = np.arange(1, 200)
signals = [np.exp(-t/20) * np.cos(t*k) for k in (1, 2)]
fig, axes = plt.subplots(nrows=2, figsize=(10, 3), layout='constrained')
axes = axes.flatten()
for i, (ax, signal) in enumerate(zip(axes, signals)):
# Plot as usual, no change to the code
ax.plot(t, signal, label=f'plotted on axes[{i}]', c='C0', lw=9, alpha=0.3)
ax2 = ax.twinx()
ax2.plot(t, signal, label=f'plotted on axes[{i}].twinx()', c='C1')
# The only specificity of the code is when plotting the legend
h, l = np.hstack([axis.get_legend_handles_labels()
for axis in ax.figure.axes
if axis.bbox.bounds == ax.bbox.bounds]).tolist()
ax2.legend(handles=h, labels=l, loc='upper right')

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