<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ggplot(df, out_file):
 """Plot faceted items with ggplot wrapper on top of matplotlib. XXX Not yet functional """

    import ggplot as gg
    df["variant.type"] = [vtype_labels[x] for x in df["variant.type"]]
    df["category"] = [cat_labels[x] for x in df["category"]]
    df["caller"] = [caller_labels.get(x, None) for x in df["caller"]]
    p = (gg.ggplot(df, gg.aes(x="caller", y="value.floor")) + gg.geom_bar()
         + gg.facet_wrap("variant.type", "category")
         + gg.theme_seaborn())
    gg.ggsave(p, out_file)