Add in HorizontalLegend.

Lists entries side-by-side in the available width, after which the
entries wrap onto the next row. If an entry is too wide it is
truncated. If there are too many entries some rows will not be shown.

Close #755.
I think normal styling rule is to avoid using ++r unless it's by itself on a line.  I don't 100% remember the order (use current r, then add?)