Do not assume the format of color parameters.

Do not assume that the `colors` parameter of the scatterplot
function is a string of one-letter color codes.

incenp/plotting/scatter.py

@@ -42,7 +42,7 @@ def scatterplot_subtrack(ax, data, n_track, n_subtrack, max_subtrack,
 
     offset = n_track * max_subtrack + n_subtrack
     xs = xdistr(data.values, width, center=True, min_sep=min_sep, offset=offset)
-    ax.plot(xs, data.values, color + '.')
+    ax.plot(xs, data.values, color=color, marker='.', linestyle='')
     ax.hlines(data.mean(), offset - .5, offset + .5, color)