Error when drawing a legend on a separate plot - python-3.x

Question

I want to plot a legend on a separate frame than the original plot. I can plot the legend from plot commands. But not the legend from fill_between.
Here is a sample code
#!/usr/bin/env python
import numpy as np
import matplotlib.pyplot as plt
xx = np.linspace(0, 3.14*3, 100)
yy = np.sin (xx)
zz = yy +0.5
# I can draw a plot with legend
fig = plt.figure( )
ax = fig.add_subplot(1, 1, 1,)
line1, = ax.plot (xx, yy, label='xx')
line2, = ax.plot (xx, zz, label='yy')
fill0 = ax.fill_between (xx, yy, zz, label='filling', alpha=0.2, color='grey' )
ax.legend ( handles=[line1, line2, fill0])
plt.show()
# I can draw a empty plot with the legend of the lines
plt.legend(handles=[line1, line2])
plt.show()
# I can't draw an empty plot with the legend of the fill
# Why ?
# Can I fix this ?
plt.legend(handles=[fill0,])
plt.show()
And now the error :
Traceback (most recent call last):
File "Untitled.py", line 34, in <module>
plt.legend(handles=[fill0,])
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/pyplot.py", line 2721, in legend
return gca().legend(*args, **kwargs)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/axes/_axes.py", line 417, in legend
self.legend_ = mlegend.Legend(self, handles, labels, **kwargs)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/legend.py", line 503, in __init__
self._init_legend_box(handles, labels, markerfirst)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/legend.py", line 767, in _init_legend_box
fontsize, handlebox))
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/legend_handler.py", line 117, in legend_artist
fontsize, handlebox.get_transform())
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib /legend_handler.py", line 727, in create_artists
self.update_prop(p, orig_handle, legend)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/legend_handler.py", line 76, in update_prop
legend._set_artist_props(legend_handle)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/legend.py", line 550, in _set_artist_props
a.set_figure(self.figure)
File "/Users/marti/anaconda3/envs/PROD/lib/python3.7/site-packages/matplotlib/artist.py", line 704, in set_figure
raise RuntimeError("Can not put single artist in "
RuntimeError: Can not put single artist in more than one figure
Any hint to fix this?

Answer 1

The artist object involved has deep nested data that matplotlib chooses not to allow its reuse on other axes. The workaround code can be something like this:
(only relevant code towards the end is given here)
# if want to plot the filled patch, must create anew
grey_patch1 = mpatches.Patch(label='filling1', alpha=0.2, color='grey')
plt.legend(handles=[line1, line2, grey_patch1])
plt.show()
#grey_patch1 can be reused, no need to recreate like this
grey_patch2 = mpatches.Patch(label='filling2', alpha=0.2, color='red')
plt.legend(handles=[grey_patch1, grey_patch2,])
plt.show()
The above code needs import matplotlib.patches as mpatches.