Matplotlib：如何绘制一个填充有颜色图的小矩形作为图例

Question

Instead of a plotting a colorbar next to my plot, I want to plot a small rectangle filled with the colormap as a legend.

I already can plot a small rectangle filled with any color, by doing the following trick:

axis0, = myax.plot([], linewidth=10, color='r')

axis =[axis0]
legend=['mytext']

plt.legend(axis,
           legend)

Can I do the same with a colormap? Thanks!

Answer 1

To my knowledge, there is no way of doing this other than by creating the rectangle and legend from scratch. Here is one approach (based mainly on this answer ):

import numpy as np                                    # v 1.19.2
import matplotlib.pyplot as plt                       # v 3.3.2
import matplotlib.patches as patches
from matplotlib.legend_handler import HandlerTuple

rng = np.random.default_rng(seed=1)

ncmaps = 5     # number of colormaps to draw for illustration
ncolors = 100  # number high enough to draw a smooth gradient for each colormap

# Create random list of colormaps and extract list of colors to 
# draw the gradient of each colormap
cmaps_names = list(rng.choice(plt.colormaps(), size=ncmaps))
cmaps = [plt.cm.get_cmap(name) for name in cmaps_names]
cmaps_gradients = [cmap(np.linspace(0, 1, ncolors)) for cmap in cmaps]
cmaps_dict = dict(zip(cmaps_names, cmaps_gradients))

# Create a list of lists of patches representing the gradient of each colormap
patches_cmaps_gradients = []
for cmap_name, cmap_colors in cmaps_dict.items():
    cmap_gradient = [patches.Patch(facecolor=c, edgecolor=c, label=cmap_name)
                     for c in cmap_colors]
    patches_cmaps_gradients.append(cmap_gradient)

# Create custom legend (with a large fontsize to better illustrate the result)
plt.legend(handles=patches_cmaps_gradients, labels=cmaps_names, fontsize=20,
           handler_map={list: HandlerTuple(ndivide=None, pad=0)})

plt.show()

If you plan on doing this for multiple plots, you may want to create a custom legend handler as shown in this answer . You may also want to consider other ways of displaying the colorbar, such as in the examples shown here and here and here .

Documentation:legend guide

Answer 2

In my CMasher package, I implemented a function called set_cmap_legend_entry() that does exactly this for you. You can give it a plot element and a label, and it will automatically create a colormap legend entry for it, like for the image shown below (see here for the documentation on this):