c_white = matplotlib.colors.colorConverter.to_rgba('white',alpha = 0)
c_black= matplotlib.colors.colorConverter.to_rgba('black',alpha = 1)
cmap_rb = matplotlib.colors.LinearSegmentedColormap.from_list('rb_cmap',[c_white,c_black],512)
pl = plt.pcolor(mp1,cmap=cmap_rb)
from matplotlib.colors import ListedColormap
cmap = np.zeros([256, 4])
cmap[:, 3] = np.linspace(0, 1, 256)
cmap = ListedColormap(cmap)