Draw plot only inside a circle with matplotlib

Question

I have scattered data and I am interpolating it to gridded data. Afterwards I draw everything with the contourf function. In the end I just want to draw everything inside a defined circle. Everything else should be white, but I don"t know how to achieve this. Is there a simple way to do this?

Here is my code:

import numpy as np
import matplotlib.pyplot as plt
from scipy.interpolate import griddata


def func(x, y):
  return x*(1-x)*np.cos(4*np.pi*x) * np.sin(4*np.pi*y**2)**2

grid_x, grid_y = np.mgrid[0:1:100j, 0:1:200j]

points = np.random.rand(1000, 2)
values = func(points[:,0], points[:,1])


grid_z0 = griddata(points, values, (grid_x, grid_y), method='nearest')

plt.contourf(grid_x,grid_y,grid_z0)

plt.savefig("plot_data_in_circle.png")

Edit: I attach the plot which comes out from my code:

And this is how it should look like:

Can you post a (MS paint) picture of what you roughly want it to look like? :) — Ivo, Apr 30 '18 at 11:53
Options you have: (a) Setting undesired values to nan, (b) mask the array, (c) use a clip path. There should be examples for that around. If not, it would be helpful to know at which point the matplotlib documentation can be improved. — ImportanceOfBeingErnest, Apr 30 '18 at 12:17
Did you try any of the above options? At which point do they fail? — ImportanceOfBeingErnest, Apr 30 '18 at 12:36
I tried

ax = plt.axes() patch = patches.Circle((0, 0), radius=0.8, transform=ax.transData) ax.set_clip_path(patch) — Jan SE, Apr 30 '18 at 12:59

tmdavison · Accepted Answer · 2018-04-30T13:23:42.793

This can be done by creating a Circle patch, and then setting that as the clip_path.

To use a clip_path, one would usually store the artist on which you want to apply the clipping, and then use .set_clip_path on that. In this case, since you are using contourf, there is no method .set_clip_path, so we have to iterate over the PathCollections stored with the QuadContourSet returned from contourf, and set the clip path on each of them using the set_clip_path method.

import numpy as np
import matplotlib.pyplot as plt
from scipy.interpolate import griddata
import matplotlib.patches as patches

def func(x, y):
  return x*(1-x)*np.cos(4*np.pi*x) * np.sin(4*np.pi*y**2)**2

grid_x, grid_y = np.mgrid[0:1:100j, 0:1:200j]

points = np.random.rand(1000, 2)
values = func(points[:,0], points[:,1])


grid_z0 = griddata(points, values, (grid_x, grid_y), method='nearest')

fig, ax = plt.subplots()
cs = ax.contourf(grid_x,grid_y,grid_z0)

circ = patches.Circle((0.6, 0.4), 0.3, transform=ax.transData)
for coll in cs.collections:
    coll.set_clip_path(circ)

plt.show()

Do you have any idea how to scale it to a circle? And I tried to include a colorbar. But this isn't working anymore — Jan SE, Apr 30 '18 at 15:14
do you mean why is it an ellipse rather than a circle? That's because the aspect ratio of the axes is not equal. Try `ax.set_aspect('equal')` and you should see a circle. — tmdavison, Apr 30 '18 at 15:23

Draw plot only inside a circle with matplotlib

1 Answers1