[FIXED] Matplotlib Wedge artists in legend

Issue

I am generating a plot with a collection of wedges using matplotlib.patches.Wedge(). The wedges are different colors and with different angles subtended. I would like to include Wedge artists in a legend, but the following MWE is resulting in Line artists in the legend instead of Wedge artists.

legend() docs show examples for Line2D and Patch artists. How can I include Wedge artists in my legend?

import matplotlib.pylab as plt
from matplotlib.patches import Wedge
from matplotlib import rc

plt.rcParams['axes.facecolor']='#EEEEEE'
plt.rcParams['savefig.facecolor']='#EEEEEE'

colors = ['#e41a1c','#377eb8','#4daf4a','#984ea3']
          
fig1 = plt.figure(figsize=(4, 3))
ax = fig1.gca().axes
radius = 0.5
legend_elements = [Wedge((0, 1), radius, 0, 90, width=radius, color=colors[0], label='category1'),
                   Wedge((0, 1), radius, 0, 180, width=radius, color=colors[1], label='category2'),
                   Wedge((0, 1), radius, 0, 270, width=radius, color=colors[2], label='category3'),
                   Wedge((0, 1), radius, 0, 360, width=radius, color=colors[3], label='category4')]
ax.legend(handles = legend_elements, loc='lower center',
          bbox_to_anchor= (0.5, 1.01), ncol=4,
          borderaxespad=0, frameon=False)
fig1.show()

Solution

You'll need to create your own Handler that tells matplotlib how to take a specific Artist and transform it to appear in a legend. Often times things need to be resized or transformed in some manner to appear better on a legend.

To create a handler, all you need to do is create any Python object that defines a legend_artist method and has the same call signature as below see docs for more info.

From there we simply create a dummy Wedge object whose radius is smaller than the box it's being drawn in (hence min(width, height)). We update the aesthetic properties on this new dummy wedge based on the inputted Artists.

Finally we add the newly created/updated dummy Wedge to the handlebox to be drawn later.

With the Handler out of the way, we now supply a handler_map to the call of ax.legend. This is essentially a dictionary that maps an Artist (in this case Wedge) to the Handler we just created, essentially tying the artist to the instructions on how to represent it in a legend.

import matplotlib.pylab as plt
from matplotlib.patches import Wedge
from matplotlib import rc

plt.rcParams['axes.facecolor']='#EEEEEE'
plt.rcParams['savefig.facecolor']='#EEEEEE'

class WedgeHandler:
    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        x0, y0 = handlebox.xdescent, handlebox.ydescent
        width, height = handlebox.width, handlebox.height
        
        handle = Wedge((0, 0), min(width, height), orig_handle.theta1, orig_handle.theta2)
        handle.update_from(orig_handle)
        handlebox.add_artist(handle)

        return handle


colors = ['#e41a1c','#377eb8','#4daf4a','#984ea3']
          
fig1 = plt.figure(figsize=(4, 3))
ax = fig1.gca().axes
radius = 0.5
legend_elements = [Wedge((0, 1), radius, 0, 90, width=radius, color=colors[0], label='category1'),
                   Wedge((0, 1), radius, 0, 180, width=radius, color=colors[1], label='category2'),
                   Wedge((0, 1), radius, 0, 270, width=radius, color=colors[2], label='category3'),
                   Wedge((0, 1), radius, 0, 360, width=radius, color=colors[3], label='category4')]
ax.legend(handles = legend_elements, loc='lower center',
          bbox_to_anchor= (0.5, 1.05), ncol=4,
          borderaxespad=0, frameon=False,
          handler_map={Wedge: WedgeHandler()}
         )

plt.show()

---

update, you may run into some issues if you use Artist.update_from as it copies over a few too many properties. Instead you can manually specify which properties you want copied using this as an example:

import matplotlib.pylab as plt
from matplotlib.patches import Wedge
from matplotlib import rc

class WedgeHandler:
    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        x0, y0 = handlebox.xdescent, handlebox.ydescent
        width, height = handlebox.width, handlebox.height
        r = min(width, height)
        
        handle = Wedge(
            center=(x0 + width / 2, y0 + height / 2),      # centers handle in handlebox
            r=r,                                           # ensures radius fits in handlebox
            width=r * (orig_handle.width / orig_handle.r), # preserves original radius/width ratio
            theta1=orig_handle.theta1,                     # copies the following parameters
            theta2=orig_handle.theta2,
            color=orig_handle.get_facecolor(),
            transform=handlebox.get_transform(),           # use handlebox coordinate system
        )
                        
        handlebox.add_artist(handle)
        return handle

plt.rcParams['axes.facecolor']='#EEEEEE'
plt.rcParams['savefig.facecolor']='#EEEEEE'

colors = ['#e41a1c','#377eb8','#4daf4a','#984ea3']
theta2 = [90, 180, 270, 360]
fig, ax = plt.subplots()

for i, (color, t2) in enumerate(zip(colors, theta2), start=1):
    wedge = Wedge((i, .5), 0.25, theta1=0, theta2=t2, width=0.25, color=color, label=f'category{i}')
    ax.add_artist(wedge)

ax.set_xlim(1-.25, i + .25)
    
legend = ax.legend(title='Default Handler', loc='upper left', bbox_to_anchor=(1.01, 1))
ax.add_artist(legend)

ax.legend(title='Custom Handler', loc='upper left', bbox_to_anchor=(1.01, 0.5), handler_map={Wedge: WedgeHandler()})
plt.show()

Answered By - Cameron Riddell