In git and snapshots there's now a trial implementation of something
that was first requested some time ago, namely independent control
of line colors and line dash-types.
If you create a plot with lines in the GUI, right-click and select
"Edit", then go to the "Lines" tab, you'll see a drop-down
selector
offering the 5 "gnuplot standard" lines (one solid and four dashed
and/or dotted patterns) for each line (up to six lines).
I haven't done a great deal of testing yet, so please bash on it and
see what happens.
Allin