Plot comparison between BAM preparation methods.

def plot_prep_methods(df, prep, prepi, out_file_base, outtype, title=None,
                      size=None):
    """Plot comparison between BAM preparation methods.
    """
    samples = df[(df["bamprep"] == prep)]["sample"].unique()
    assert len(samples) >= 1, samples
    out_file = "%s-%s.%s" % (out_file_base, samples[0], outtype)
    df = df[df["category"].isin(cat_labels)]
    _seaborn(df, prep, prepi, out_file, title, size)
    return out_file