Plot paired scatterplots and compute RPM.

Made paired scatterplot plotting function part of libhts as
`plot_paired_scatters`. Modifications in small RNA-seq pipeline not
tested.

libhts/__init__.py

-from .libhts import do_deseq2, gtf_2_genes_exon_lengths, median_ratio_to_pseudo_ref_size_factors, plot_boxplots, plot_counts_distribution, plot_lfc_distribution, plot_MA, plot_norm_correlations, plot_scatter, repeat_bed_2_lengths, size_factor_correlations, spikein_gtf_2_lengths, status_setter
+from .libhts import do_deseq2, gtf_2_genes_exon_lengths, median_ratio_to_pseudo_ref_size_factors, plot_boxplots, plot_counts_distribution, plot_lfc_distribution, plot_MA, plot_norm_correlations, plot_paired_scatters, plot_scatter, repeat_bed_2_lengths, size_factor_correlations, spikein_gtf_2_lengths, status_setter

libhts/libhts.py

@@ -506,3 +506,12 @@ def plot_scatter(data,
         else:
             ax.set_ylim(y_range)
     return ax
+
+def plot_paired_scatters(data, columns=None, hue=None):
+    """Alternative to pairplot, in order to avoid histograms on the diagonal."""
+    if columns is None:
+        columns = data.columns
+    g = sns.PairGrid(data, vars=columns, hue=hue, size=8)
+    #g.map_offdiag(plt.scatter, marker=".")
+    g.map_lower(plt.scatter, marker=".")
+    g.add_legend()