[matplotlib-devel] scatter and alpha

I found that in scatter plots the alpha values given by individual 
entries in the color list are ignored. Here an example that shows 
different combinations of color and alpha arguments:

x = [1,2,3]
y = array([1,1,1])

c = [[1,0,0, 0.0],
     [1,0,0, 0.5],
     [1,0,0, 1.0]]

scatter(x, y, s = 200, c = 'r')
scatter(x, y + 1, s = 200, c = c)
scatter(x, y + 2, s = 200, c = c, alpha = 0.5)
scatter(x, y + 3, s = 200, c = 'g', alpha = 0.5)

Looking at the source in axes.py/scatter I found that replacing the 
default value for the alpha keyword argument in the function header of 
scatter from alpha=1.0 to alpha=None fixes this problem. Additionally, I 
had to replace

collection.set_alpha(alpha)

by

if alpha is not None:
    collection.set_alpha(alpha)

See also the attached diff file.
 
Gregor