Draws a subtle vertical gradient strip.

def vertical_strip(width=10, height=100, color=rgb(100, 100, 100),
                   subtlety=0.1):
    """
    Draws a subtle vertical gradient strip.
    """

    cairo_color = color / rgb(255, 255, 255)

    surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
    ctx = cairo.Context(surface)

    ctx.scale(width / 1.0, height / 1.0)

    pat = cairo.LinearGradient(0.0, 0.0, 0.0, 1.0)
    pat.add_color_stop_rgba(
        0,
        cairo_color.red,
        cairo_color.green,
        cairo_color.blue,
        0
    )
    pat.add_color_stop_rgba(
        1,
        cairo_color.red,
        cairo_color.green,
        cairo_color.blue,
        1
    )

    ctx.rectangle(0, 0, 1, 1)
    ctx.set_source(pat)
    ctx.fill()

    surface.write_to_png('vertical_strip.png')