def gff(args):
    """
    %prog gff *.gff

    Draw exons for genes based on gff files. Each gff file should contain only
    one gene, and only the "mRNA" and "CDS" feature will be drawn on the canvas.
    """
    align_choices = ("left", "center", "right")
    p = OptionParser(gff.__doc__)
    p.add_option("--align", default="left", choices=align_choices,
                 help="Horizontal alignment [default: %default]")
    p.add_option("--noUTR", default=False, action="store_true",
                 help="Do not plot UTRs [default: %default]")
    opts, args = p.parse_args(args)

    if len(args) < 1:
        sys.exit(not p.print_help())

    fig = plt.figure(1, (8, 5))
    root = fig.add_axes([0, 0, 1, 1])

    gffiles = args
    ngenes = len(gffiles)

    canvas = .6
    setups, ratio = get_setups(gffiles, canvas=canvas, noUTR=opts.noUTR)
    align = opts.align
    xs = .2 if align == "left" else .8
    yinterval = canvas / ngenes
    ys = .8
    tip = .01
    for genename, mrnabed, cdsbeds in setups:
        ExonGlyph(root, xs, ys, mrnabed, cdsbeds, ratio=ratio, align=align)
        if align == "left":
            root.text(xs - tip, ys, genename, ha="right", va="center")
        elif align == "right":
            root.text(xs + tip, ys, genename, ha="left", va="center")
        ys -= yinterval

    root.set_xlim(0, 1)
    root.set_ylim(0, 1)
    root.set_axis_off()

    figname = "exons.pdf"
    savefig(figname, dpi=300)