def ld(args):
    """
    %prog ld map

    Calculate pairwise linkage disequilibrium given MSTmap.
    """
    import numpy as np
    from random import sample

    from jcvi.algorithms.matrix import symmetrize

    p = OptionParser(ld.__doc__)
    p.add_option("--subsample", default=1000, type="int",
                 help="Subsample markers to speed up [default: %default]")
    opts, args, iopts = p.set_image_options(args, figsize="8x8")

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

    mstmap, = args
    subsample = opts.subsample
    data = MSTMap(mstmap)

    markerbedfile = mstmap + ".subsample.bed"
    ldmatrix = mstmap + ".subsample.matrix"
    # Take random subsample while keeping marker order
    if data.nmarkers < subsample:
        data = [data[x] for x in \
                sorted(sample(xrange(len(data)), subsample))]
    else:
        logging.debug("Use all markers, --subsample ignored")

    nmarkers = len(data)
    if need_update(mstmap, (ldmatrix, markerbedfile)):
        fw = open(markerbedfile, "w")
        print >> fw, "\n".join(x.bedline for x in data)
        logging.debug("Write marker set of size {0} to file `{1}`."\
                        .format(nmarkers, markerbedfile))
        fw.close()

        M = np.zeros((nmarkers, nmarkers), dtype=float)
        for i, j in combinations(range(nmarkers), 2):
            a = data[i]
            b = data[j]
            M[i, j] = calc_ldscore(a.genotype, b.genotype)

        M = symmetrize(M)

        logging.debug("Write LD matrix to file `{0}`.".format(ldmatrix))
        M.tofile(ldmatrix)
    else:
        nmarkers = len(Bed(markerbedfile))
        M = np.fromfile(ldmatrix, dtype="float").reshape(nmarkers, nmarkers)
        logging.debug("LD matrix `{0}` exists ({1}x{1})."\
                        .format(ldmatrix, nmarkers))

    from jcvi.graphics.base import plt, savefig, Rectangle, draw_cmap

    plt.rcParams["axes.linewidth"] = 0

    fig = plt.figure(1, (iopts.w, iopts.h))
    root = fig.add_axes([0, 0, 1, 1])
    ax = fig.add_axes([.1, .1, .8, .8])  # the heatmap

    ax.matshow(M, cmap=iopts.cmap)

    # Plot chromosomes breaks
    bed = Bed(markerbedfile)
    xsize = len(bed)
    extent = (0, nmarkers)
    chr_labels = []
    ignore_size = 20

    for (seqid, beg, end) in bed.get_breaks():
        ignore = abs(end - beg) < ignore_size
        pos = (beg + end) / 2
        chr_labels.append((seqid, pos, ignore))
        if ignore:
            continue
        ax.plot((end, end), extent, "w-", lw=1)
        ax.plot(extent, (end, end), "w-", lw=1)

    # Plot chromosome labels
    for label, pos, ignore in chr_labels:
        pos = .1 + pos * .8 / xsize
        if not ignore:
            root.text(pos, .91, label,
                ha="center", va="bottom", rotation=45, color="grey")
            root.text(.09, pos, label,
                ha="right", va="center", color="grey")

    ax.set_xlim(extent)
    ax.set_ylim(extent)
    ax.set_axis_off()

    draw_cmap(root, "Pairwise LD (r2)", 0, 1, cmap=iopts.cmap)

    root.add_patch(Rectangle((.1, .1), .8, .8, fill=False, ec="k", lw=2))
    m = mstmap.split(".")[0]
    root.text(.5, .06, "Linkage Disequilibrium between {0} markers".format(m), ha="center")

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

    image_name = m + ".subsample" + "." + iopts.format
    savefig(image_name, dpi=iopts.dpi, iopts=iopts)
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