gnuplot development

Currently (4.2.0), gnuplot automatically determines whether it should
create a new column/row in the key (legend).

However, sometimes it would be nice if the user could control the number of
columns/rows.

I don't care whether this is a column <n> / row <n> option or an width
<size> / height <size> option.

This option is useful for multiplot when the key should be wider than the
size of a single plot; example:

+------------+ +------------+
| Fig 1 | | Fig 2 |
| same plot | | split in |
| two regions| | |
+------------+ +------------+
Common key spreading over
both plots and having
multiple columns.

The placing in multiplot works ok, but setting the width does not work. But
also for non-multiplot plots it has its use.