Update gnuplot to version 5.4.3 / rev 85 via SR 983854
    
    https://build.opensuse.org/request/show/983854
    by user WernerFink + dimstar_suse
    - Use wxGTK3 instead of GTK2 wxWidgets.
    - Update to gnuplot 5.4.3
      * NEW "set pm3d border retrace" will use quadrangle color for the border
      * NEW "nogrid" flag for plots to opt out of dgrid3d processing
      * NEW option "above|below y=<val>" for "plot with fillsteps"
      * NEW "set style histogram nokeyseparators" suppress blank key entries Bug #2473
      * CHANGE always allow palette color for non-plot elements of 2D plot
      * FIX contours were incorrectly allocated space in the key even if "notitle"
      * FIX evaluation of unary minus in front of ternary operator Bug #2468
      * FIX arm and s390 require "signed char" Bugs #2467 #2450
      * FIX range errors if "smooth csplines" applied with x-axis reversed Bug #2465
      * FIX handle various corner cases found by fuzzing
      * FIX textcolor should accept