Custom colormap to map low values to black or another color.

def diff_colormap():
    "Custom colormap to map low values to black or another color."

    # bottom = plt.cm.copper(np.linspace(0., 1, 6))
    black = np.atleast_2d([0., 0., 0., 1.])
    bottom = np.repeat(black, 6, axis=0)
    middle = plt.cm.copper(np.linspace(0, 1, 250))
    # remain = plt.cm.Reds(np.linspace(0, 1, 240))

    colors = np.vstack((bottom, middle))
    diff_colormap = mpl.colors.LinearSegmentedColormap.from_list('diff_colormap', colors)

    return diff_colormap