[Matplotlib-users] Single color transparent colormap

[Loic E. <loi...@in...>]

Dear all, 

I am trying to create a colormap with a single color (red in the example below) where the alpha varies from 0 to 1. It does look like I am getting some grayish color near the low alpha values (around alpha = 0.2). Is that expected somehow? 

The plot I get is here: 
https://cloud.githubusercontent.com/assets/1680079/5084457/7d2d3790-6f06-11e4-9021-5b9e77e6a9c4.png 

I am using matplotlib 1.4.2. 

Here is a snippet which reproduces the issue. 

import numpy as np 

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

import matplotlib 

matplotlib.rcParams['figure.facecolor'] = 'white' 

cm_dict = {'red': ((0.0, 1.0, 1.0), 
(1.0, 1.0, 1.0)), 
'green': ((0.0, 0.0, 0.0), 
(1.0, 0.0, 0.0)), 
'blue': ((0.0, 0.0, 0.0), 
(1.0, 0.0, 0.0)), 
'alpha': ((0.0, 0.0, 0.0), 
(1.0, 1.0, 1.0)) 
} 

my_cm = LinearSegmentedColormap('my_cm', cm_dict) 

vals = np.tile(np.linspace(-1, 1, 30), (20, 1)) 

fig = plt.figure() 
ax = plt.imshow(vals, cmap=my_cm) 
plt.colorbar() 
plt.show() 

Cheers, 
Loïc