[Matplotlib-users] Clipping a plot inside a polygon

[Alex G. <ale...@co...>]

Hi all,

I want to be able to plot data on maps (using basemap or cartopy) inside
specific regions, eg a single state, province or country. A similar
question was asked a long time ago on the mailing list and the suggested
solution back then was to read the bounding polygon from a shapefile and
then check if each individual point was inside that polygon. Currently I
have no problem doing this if I use matplotlib.path.Path.contains_points()
to mask the original data array, but the disadvantage to this solution is
that it is very slow. Another solution that I have discovered recently is
to use the set_clip_path() method for artists. In addition to being much
faster, this also makes the areas near the polygon boundary look much
smoother since the actual items being clipped are individual pixels and not
data points.

Here is an example script that plots an image via imshow, but the only part
of the image that gets shown is inside the hexagon.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.patches import RegularPolygon

data = np.arange(100).reshape(10, 10)
fig = plt.figure()
ax = fig.add_subplot(111)
im = ax.imshow(data)
poly = RegularPolygon([ 0.5,  0.5], 6, 0.4, fc='none',
  ec='k', transform=ax.transAxes)
im.set_clip_path(poly)
ax.add_patch(poly)
ax.axis('off')
plt.show()

While this does seem like an ideal solution, it doesn't work for every type
of plot. The most notable example is contourf(). It returns a
QuadContourSet instance which does not inherit from Artist, so it does not
contain the set_clip_path() method. My main question is whether there is a
mechanism in matplotlib that can convert something like a QuadContourSet
into an image so I can make use of this solution for contourf() as well. Or
better yet, is there perhaps another artist within the axes that I can use
the set_clip_path() method for and still get what I want?

Thanks,
Alex
-- 
Alex Goodman
Graduate Research Assistant
Department of Atmospheric Science
Colorado State University