Legend guide#
This legend guide is an extension of the documentation available at legend() — please ensure you are familiar with contents of that documentation before proceeding with this guide.
This guide makes use of some common terms, which are documented here for clarity:
A legend is made up of one or more legend entries. An entry is made up of exactly one key and one label.
The colored/patterned marker to the left of each legend label.
The text which describes the handle represented by the key.
The original object which is used to generate an appropriate entry in the legend.
Controlling the legend entries#
Calling legend() with no arguments automatically fetches the legend handles and their associated labels. This functionality is equivalent to:
handles, labels = ax.get_legend_handles_labels() ax.legend(handles, labels)
The get_legend_handles_labels() function returns a list of handles/artists which exist on the Axes which can be used to generate entries for the resulting legend — it is worth noting however that not all artists can be added to a legend, at which point a «proxy» will have to be created (see Creating artists specifically for adding to the legend (aka. Proxy artists) for further details).
Artists with an empty string as label or with a label starting with an underscore, «_», will be ignored.
For full control of what is being added to the legend, it is common to pass the appropriate handles directly to legend() :
fig, ax = plt.subplots() line_up, = ax.plot([1, 2, 3], label='Line 2') line_down, = ax.plot([3, 2, 1], label='Line 1') ax.legend(handles=[line_up, line_down])
In some cases, it is not possible to set the label of the handle, so it is possible to pass through the list of labels to legend() :
fig, ax = plt.subplots() line_up, = ax.plot([1, 2, 3], label='Line 2') line_down, = ax.plot([3, 2, 1], label='Line 1') ax.legend([line_up, line_down], ['Line Up', 'Line Down'])
Creating artists specifically for adding to the legend (aka. Proxy artists)#
Not all handles can be turned into legend entries automatically, so it is often necessary to create an artist which can. Legend handles don’t have to exist on the Figure or Axes in order to be used.
Suppose we wanted to create a legend which has an entry for some data which is represented by a red color:
import matplotlib.patches as mpatches import matplotlib.pyplot as plt fig, ax = plt.subplots() red_patch = mpatches.Patch(color='red', label='The red data') ax.legend(handles=[red_patch]) plt.show()
There are many supported legend handles. Instead of creating a patch of color we could have created a line with a marker:
import matplotlib.lines as mlines fig, ax = plt.subplots() blue_line = mlines.Line2D([], [], color='blue', marker='*', markersize=15, label='Blue stars') ax.legend(handles=[blue_line]) plt.show()
Legend location#
The location of the legend can be specified by the keyword argument loc. Please see the documentation at legend() for more details.
The bbox_to_anchor keyword gives a great degree of control for manual legend placement. For example, if you want your axes legend located at the figure’s top right-hand corner instead of the axes’ corner, simply specify the corner’s location and the coordinate system of that location:
ax.legend(bbox_to_anchor=(1, 1), bbox_transform=fig.transFigure)
More examples of custom legend placement:
fig, ax_dict = plt.subplot_mosaic([['top', 'top'], ['bottom', 'BLANK']], empty_sentinel="BLANK") ax_dict['top'].plot([1, 2, 3], label="test1") ax_dict['top'].plot([3, 2, 1], label="test2") # Place a legend above this subplot, expanding itself to # fully use the given bounding box. ax_dict['top'].legend(bbox_to_anchor=(0., 1.02, 1., .102), loc='lower left', ncols=2, mode="expand", borderaxespad=0.) ax_dict['bottom'].plot([1, 2, 3], label="test1") ax_dict['bottom'].plot([3, 2, 1], label="test2") # Place a legend to the right of this smaller subplot. ax_dict['bottom'].legend(bbox_to_anchor=(1.05, 1), loc='upper left', borderaxespad=0.)
Figure legends#
Sometimes it makes more sense to place a legend relative to the (sub)figure rather than individual Axes. By using constrained_layout and specifying «outside» at the beginning of the loc keyword argument, the legend is drawn outside the Axes on the (sub)figure.
fig, axs = plt.subplot_mosaic([['left', 'right']], layout='constrained') axs['left'].plot([1, 2, 3], label="test1") axs['left'].plot([3, 2, 1], label="test2") axs['right'].plot([1, 2, 3], 'C2', label="test3") axs['right'].plot([3, 2, 1], 'C3', label="test4") # Place a legend to the right of this smaller subplot. fig.legend(loc='outside upper right')
This accepts a slightly different grammar than the normal loc keyword, where «outside right upper» is different from «outside upper right».
ucl = ['upper', 'center', 'lower'] lcr = ['left', 'center', 'right'] fig, ax = plt.subplots(figsize=(6, 4), layout='constrained', facecolor='0.7') ax.plot([1, 2], [1, 2], label='TEST') # Place a legend to the right of this smaller subplot. for loc in [ 'outside upper left', 'outside upper center', 'outside upper right', 'outside lower left', 'outside lower center', 'outside lower right']: fig.legend(loc=loc, title=loc) fig, ax = plt.subplots(figsize=(6, 4), layout='constrained', facecolor='0.7') ax.plot([1, 2], [1, 2], label='test') for loc in [ 'outside left upper', 'outside right upper', 'outside left lower', 'outside right lower']: fig.legend(loc=loc, title=loc)
Multiple legends on the same Axes#
Sometimes it is more clear to split legend entries across multiple legends. Whilst the instinctive approach to doing this might be to call the legend() function multiple times, you will find that only one legend ever exists on the Axes. This has been done so that it is possible to call legend() repeatedly to update the legend to the latest handles on the Axes. To keep old legend instances, we must add them manually to the Axes:
fig, ax = plt.subplots() line1, = ax.plot([1, 2, 3], label="Line 1", linestyle='--') line2, = ax.plot([3, 2, 1], label="Line 2", linewidth=4) # Create a legend for the first line. first_legend = ax.legend(handles=[line1], loc='upper right') # Add the legend manually to the Axes. ax.add_artist(first_legend) # Create another legend for the second line. ax.legend(handles=[line2], loc='lower right') plt.show()
Legend Handlers#
In order to create legend entries, handles are given as an argument to an appropriate HandlerBase subclass. The choice of handler subclass is determined by the following rules:
- Update get_legend_handler_map() with the value in the handler_map keyword.
- Check if the handle is in the newly created handler_map .
- Check if the type of handle is in the newly created handler_map .
- Check if any of the types in the handle ‘s mro is in the newly created handler_map .
For completeness, this logic is mostly implemented in get_legend_handler() .
All of this flexibility means that we have the necessary hooks to implement custom handlers for our own type of legend key.
The simplest example of using custom handlers is to instantiate one of the existing legend_handler.HandlerBase subclasses. For the sake of simplicity, let’s choose legend_handler.HandlerLine2D which accepts a numpoints argument (numpoints is also a keyword on the legend() function for convenience). We can then pass the mapping of instance to Handler as a keyword to legend.
from matplotlib.legend_handler import HandlerLine2D fig, ax = plt.subplots() line1, = ax.plot([3, 2, 1], marker='o', label='Line 1') line2, = ax.plot([1, 2, 3], marker='o', label='Line 2') ax.legend(handler_map=line1: HandlerLine2D(numpoints=4)>)
As you can see, «Line 1» now has 4 marker points, where «Line 2» has 2 (the default). Try the above code, only change the map’s key from line1 to type(line1) . Notice how now both Line2D instances get 4 markers.
Along with handlers for complex plot types such as errorbars, stem plots and histograms, the default handler_map has a special tuple handler ( legend_handler.HandlerTuple ) which simply plots the handles on top of one another for each item in the given tuple. The following example demonstrates combining two legend keys on top of one another:
from numpy.random import randn z = randn(10) fig, ax = plt.subplots() red_dot, = ax.plot(z, "ro", markersize=15) # Put a white cross over some of the data. white_cross, = ax.plot(z[:5], "w+", markeredgewidth=3, markersize=15) ax.legend([red_dot, (red_dot, white_cross)], ["Attr A", "Attr A+B"])
The legend_handler.HandlerTuple class can also be used to assign several legend keys to the same entry:
from matplotlib.legend_handler import HandlerLine2D, HandlerTuple fig, ax = plt.subplots() p1, = ax.plot([1, 2.5, 3], 'r-d') p2, = ax.plot([3, 2, 1], 'k-o') l = ax.legend([(p1, p2)], ['Two keys'], numpoints=1, handler_map=tuple: HandlerTuple(ndivide=None)>)
Implementing a custom legend handler#
A custom handler can be implemented to turn any handle into a legend key (handles don’t necessarily need to be matplotlib artists). The handler must implement a legend_artist method which returns a single artist for the legend to use. The required signature for legend_artist is documented at legend_artist .
import matplotlib.patches as mpatches class AnyObject: pass class AnyObjectHandler: def legend_artist(self, legend, orig_handle, fontsize, handlebox): x0, y0 = handlebox.xdescent, handlebox.ydescent width, height = handlebox.width, handlebox.height patch = mpatches.Rectangle([x0, y0], width, height, facecolor='red', edgecolor='black', hatch='xx', lw=3, transform=handlebox.get_transform()) handlebox.add_artist(patch) return patch fig, ax = plt.subplots() ax.legend([AnyObject()], ['My first handler'], handler_map=AnyObject: AnyObjectHandler()>)
Alternatively, had we wanted to globally accept AnyObject instances without needing to manually set the handler_map keyword all the time, we could have registered the new handler with:
from matplotlib.legend import Legend Legend.update_default_handler_map(AnyObject: AnyObjectHandler()>)
Whilst the power here is clear, remember that there are already many handlers implemented and what you want to achieve may already be easily possible with existing classes. For example, to produce elliptical legend keys, rather than rectangular ones:
from matplotlib.legend_handler import HandlerPatch class HandlerEllipse(HandlerPatch): def create_artists(self, legend, orig_handle, xdescent, ydescent, width, height, fontsize, trans): center = 0.5 * width - 0.5 * xdescent, 0.5 * height - 0.5 * ydescent p = mpatches.Ellipse(xy=center, width=width + xdescent, height=height + ydescent) self.update_prop(p, orig_handle, legend) p.set_transform(trans) return [p] c = mpatches.Circle((0.5, 0.5), 0.25, facecolor="green", edgecolor="red", linewidth=3) fig, ax = plt.subplots() ax.add_patch(c) ax.legend([c], ["An ellipse, not a rectangle"], handler_map=mpatches.Circle: HandlerEllipse()>)
Total running time of the script: ( 0 minutes 4.644 seconds)
Adding a matplotlib legend
How can one create a legend for a line graph in Matplotlib ‘s PyPlot without creating any extra variables? Please consider the graphing script below:
if __name__ == '__main__': PyPlot.plot(total_lengths, sort_times_bubble, 'b-', total_lengths, sort_times_ins, 'r-', total_lengths, sort_times_merge_r, 'g+', total_lengths, sort_times_merge_i, 'p-', ) PyPlot.title("Combined Statistics") PyPlot.xlabel("Length of list (number)") PyPlot.ylabel("Time taken (seconds)") PyPlot.show() 
As you can see, this is a very basic use of matplotlib ‘s PyPlot . This ideally generates a graph like the one below: Nothing special, I know. However, it is unclear what data is being plotted where (I’m trying to plot the data of some sorting algorithms, length against time taken, and I’d like to make sure people know which line is which). Thus, I need a legend, however, taking a look at the following example below(from the official site):
ax = subplot(1,1,1) p1, = ax.plot([1,2,3], label="line 1") p2, = ax.plot([3,2,1], label="line 2") p3, = ax.plot([2,3,1], label="line 3") handles, labels = ax.get_legend_handles_labels() # reverse the order ax.legend(handles[::-1], labels[::-1]) # or sort them by labels import operator hl = sorted(zip(handles, labels), key=operator.itemgetter(1)) handles2, labels2 = zip(*hl) ax.legend(handles2, labels2) You will see that I need to create an extra variable ax . How can I add a legend to my graph without having to create this extra variable and retaining the simplicity of my current script?