set style increment {default|userstyles} show style increment
By default, successive plots within the same graph will use successive linetypes from the default set for the current terminal type. However, choosing set style increment user allows you to step through the user-defined line styles rather than through the default linetypes.
Example:
set style line 1 lw 2 lc rgb "gold" set style line 2 lw 2 lc rgb "purple" set style line 4 lw 1 lc rgb "sea-green" set style increment user
plot f1(x), f2(x), f3(x), f4(x)
should plot functions f1, f2, f4 in your 3 newly defined line styles. If a user-defined line style is not found then the corresponding default linetype is used instead. E.g. in the example above, f3(x) will be plotted using the default linetype 3.