def select_color_marker(i):
    """ Return index-based marker/color format for plotting """
    colors = ['b', 'g', 'r', 'c', 'y', 'k']
    style = ['-', '--', '-.', ':']
    ci = i % len(colors)
    si = (i // len(colors)) % len(style)
    return '%s%s' % (colors[ci], style[si])