bugfix: handle alpha correctly in colormapping with uint8

svn path=/trunk/matplotlib/; revision=8968

Author: efiring

lib/matplotlib/colors.py

@@ -547,6 +547,8 @@ def __call__(self, X, alpha=None, bytes=False):
         if alpha is not None:
             alpha = min(alpha, 1.0) # alpha must be between 0 and 1
             alpha = max(alpha, 0.0)
+            if bytes:
+                alpha = int(alpha * 255)
             if (lut[-1] == 0).all():
                 lut[:-1, -1] = alpha
                 # All zeros is taken as a flag for the default bad