Gnuplot version 4.0 deprecated certain syntax used in earlier versions, but continued to recognize it. This is now under the control of a configuration option, and can be disabled as follows:
Notice: Deprecated syntax items may be disabled permanently in some future version of gnuplot.
One major difference is the introduction of keywords to disambiguate complex commands, particularly commands containing string variables. A notable issue was the use of bare numbers to specify offsets, line and point types. Illustrative examples:
set title "Old" 0,-1 set data linespoints plot 1 2 4 # horizontal line at y=1
TITLE = "New" set title TITLE offset char 0, char -1 set style data linespoints plot 1 linetype 2 pointtype 4