Re: [Matplotlib-users] scatter plot individual alpha values

[Tony Yu <ts...@gm...>]

On Wed, Aug 8, 2012 at 4:34 PM, Gustavo Goretkin <gus...@gm...
> wrote:

> I can use the scatter function to plot an array of points and give a
> corresponding array of colors to set those points. Is it possible to
> do the same thing with alpha values?
>
> Right now, I'm restoring to calling plot with an 'o' marker for each
> individual point, which is slow.
>
>
You can use a colormap with varying alpha values.

Example below. I'm not a huge fan of the call signature of
`LinearSegmentedColormap`, so I've included a class that makes it a little
more convenient to define the colormap.

Cheers,
-Tony

P.S. There were some major cleanups of the alpha handling not too long ago,
but I don't think those are necessary for scatter. Let me know if this
doesn't work for some reason.

#~~~ example code

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap


class LinearColormap(LinearSegmentedColormap):

    def __init__(self, name, segmented_data, index=None, **kwargs):
        if index is None:
            # If index not given, RGB colors are evenly-spaced in colormap.
            index = np.linspace(0, 1, len(segmented_data['red']))
        for key, value in segmented_data.iteritems():
            # Combine color index with color values.
            segmented_data[key] = zip(index, value)
        segmented_data = dict((key, [(x, y, y) for x, y in value])
                              for key, value in segmented_data.iteritems())
        LinearSegmentedColormap.__init__(self, name, segmented_data,
**kwargs)


# Red for all values, but alpha changes linearly from 0.3 to 1
color_spec = {'blue':  [0.0, 0.0],
           'green': [0.0, 0.0],
           'red':   [0.8, 0.8],
           'alpha': [0.3, 1.0]}
alpha_red = LinearColormap('alpha_red', color_spec)

x, y, z = np.random.normal(size=(3, 100))

plt.scatter(x, y, c=z, cmap=alpha_red, s=50, edgecolors='none')
plt.show()


# Here's a slightly more complicated use of LinearColormap, if you're
interested.

# Blue below midpoint of colormap, red above mid point.
# Alpha maximum at the edges, minimum in the middle.
bwr_spec = {'blue': [0.4, 0.4, 0.1, 0.1],
           'green': [0.2, 0.2, 0.0, 0.0],
           'red': [0.02, 0.02, 0.4, 0.4],
           'alpha': [1, 0.3, 0.3, 1]}
blue_white_red = LinearColormap('blue_white_red', bwr_spec,
                                index=[0, 0.5, 0.5, 1])