Add item to existing Matplotlib legend

Question

Given the following setup:

from matplotlib import pyplot as plt
fig, ax = plt.subplots()
ax.plot([0,1,2,3,4,5,6], label='linear')
ax.plot([0,1,4,9,16,25,36], label='square')
lgd = ax.legend(loc='lower right')

If a function add_patch receives only lgd as an argument, can a custom legend item be added to the legend on top of the existing items, without changing the other properties of the legend?

I was able to add an item using:

def add_patch(legend):
    from matplotlib.patches import Patch
    ax = legend.axes
    handles, labels = ax.get_legend_handles_labels()
    handles.append(Patch(facecolor='orange', edgecolor='r'))
    labels.append("Color Patch")
    ax.legend(handles=handles, labels=labels)

But this does not preserve the properties of the legend like location. How can I add an item given only the legend object after lines have been plotted?

Answer 1

In principle a legend is not meant to be updated, but recreated instead.

The following would do what you're after, but beware that this is a hack which uses internal methods and is hence not guaranteed to work and might break in future releases. So don't use it in production code. Also, if you have set a title to the legend with a different font(size) than default, it will be lost upon updating. Also, if you have manipulated the order of handles and labels via markerfirst, this will be lost upon updating.

from matplotlib import pyplot as plt
fig, ax = plt.subplots()
ax.plot([0,1,2,3,4,5,6], label='linear')
ax.plot([0,1,4,9,16,25,36], label='square')
lgd = ax.legend(loc='lower right')

def add_patch(legend):
    from matplotlib.patches import Patch
    ax = legend.axes

    handles, labels = ax.get_legend_handles_labels()
    handles.append(Patch(facecolor='orange', edgecolor='r'))
    labels.append("Color Patch")

    legend._legend_box = None
    legend._init_legend_box(handles, labels)
    legend._set_loc(legend._loc)
    legend.set_title(legend.get_title().get_text())


add_patch(lgd)

plt.show()

Answer 2

Is adding the color patch after the lines have been plotted but before adding the legend an option?

import matplotlib.pyplot as plt
from matplotlib.patches import Patch

fig, ax = plt.subplots()
line1 = ax.plot([0,1,2,3,4,5,6], label='linear')
line2 = ax.plot([0,1,4,9,16,25,36], label='square')
patch = Patch(facecolor='orange', edgecolor='r', label='Color patch')
lgd = ax.legend(handles=[line1, line2, patch], loc='lower right')