set contour
set title "Demo of specifying discrete contour levels  -  default contours"
splot x*y

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#set discrete levels
set cntrparam levels discrete 0, 15, 75
set title "3 discrete contours at 0 15 & 75"
replot

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#set incremental levels
set cntrp level incr -20, 5, 9
set title "9 incremental contours starting at -20, stepping by 5"
replot

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#restore defaults
set title ""
unset contour
set cntrparam levels auto 5