get_facecolors() reports incorrect colors

[nschloe]

MWE:

import matplotlib
from matplotlib import pyplot as plt
import numpy as np

fig = plt.figure()

N = 20
x = np.random.rand(N)
y = np.random.rand(N)
colors = np.random.rand(N)
area = np.pi * (15 * np.random.rand(N))**2
plt.scatter(x, y, s=area, c=colors, alpha=0.5)

children = fig.get_children()
for child in children:
    if isinstance(child, matplotlib.axes.Subplot):
        cc = child.get_children()
        print(cc[0].get_facecolors())

plt.show()

Output:

[[ 0.   0.   1.   0.5]]

[WeatherGod]

This is a similar problem to one that has been found in mplot3d. The issue
is that for ScalarMappables like scatter plots, the colors aren't set until
plotting, so they start off with the default polygon/patch color.

On Thu, Oct 20, 2016 at 9:00 AM, Nico Schlömer notifications@github.com
wrote:

MWE:

import matplotlibfrom matplotlib import pyplot as pltimport numpy as np

fig = plt.figure()
N = 20
x = np.random.rand(N)
y = np.random.rand(N)
colors = np.random.rand(N)
area = np.pi * (15 * np.random.rand(N))**2
plt.scatter(x, y, s=area, c=colors, alpha=0.5)

children = fig.get_children()for child in children:
if isinstance(child, matplotlib.axes.Subplot):
cc = child.get_children()
print(cc[0].get_facecolors())

plt.show()

Output:

[[ 0. 0. 1. 0.5]]

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[nschloe]

Mhm. Any other ways of retrieving the colors from the figure then?

[tacaswell]

Why are you trying to do this?

To expand on what @WeatherGod said, the reason that the colors are not set until late is to avoid extra computation as you can change the underlying data, norm and colormap of the ScalarMappable.

[nschloe]

Why are you trying to do this?

This is for matplotlib2tikz, in particular nschloe/tikzplotlib#120.

Perhaps there's an mpl function then that returns the color data from the figure?

[anntzer]

Actually this issue is solved as of 2.0b4 (I did check that I could reproduce it with 1.5.3).

[nschloe]

@anntzer Tried this with 2.0b4 and got

[[ 0.12156863  0.46666667  0.70588235  0.5       ]]

This is still one entry for 20 circles. Do you get the same? If yes, I guess a reopen is in order.

[anntzer]

Oh, I see. I saw that the color was not 0, 0, 1, .5 anymore and thought that was enough, sorry about the premature closing.

[anntzer]

You can (should?) call fig.canvas.draw() first. Then the facecolors become correctly set.

[nschloe]

Indeed, when inserting fig.canvas.draw() I get

[[ 0.9553    0.901065  0.118128  0.5     ]
 [ 0.120092  0.600104  0.54253   0.5     ]
 # ...
 [ 0.945636  0.899815  0.112838  0.5     ]]

which are the actual sizes after colormap matching. Better would be to get the values of the c argument of the scatter call, much like get_sizes() returns the s argument. Any way to do that?

[anntzer]

Ah. For that, you have cc[0].get_array().

[nschloe]

Just what I was looking for. Thanks!