While trying to move to wxGTK3 I saw that gnuplot is supposed to work (as is working in that way in debian for some time) with wxGTK3 instead of the old 2.8 slot. In the following bug report you will see that the bug was fixed at 2014 and 5.0 branch included the fix, being 5.0.1 released at this year and including it finally: http://sourceforge.net/p/gnuplot/bugs/1401/ Thanks
[master 3dd6502] sci-visualization/gnuplot: Support wxGTK:3.0 (#562486) 2 files changed, 216 insertions(+) create mode 100644 sci-visualization/gnuplot/files/gnuplot-5.0.1-fix-underlinking.patch create mode 100644 sci-visualization/gnuplot/gnuplot-5.0.1-r1.ebuild