#2990 Line styles with gnuplot 5.0 and plot

[Gunter Königsmann]

To make linestyles work with gnuplot versions => 5.0 the following lines have to be added to the preamble:

if(GPVAL_VERSION >= 5.0){set for [i=1:8] linetype i dashtype i}

Did already add a patch for draw - but plot is much more complex.

Kind regards,

   Gunter.

[Gunter Königsmann]

Did commit the patch for draw to the repo. But plot still is more lisp magic than I am secure I am able to do.