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:
./configure --disable-backwards-compatibility
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:
Deprecated:
set title "Old" 0,-1
set data linespoints
plot 1 2 4 # horizontal line at y=1
New:
TITLE = "New"
set title TITLE offset char 0, char -1
set style data linespoints
plot 1 linetype 2 pointtype 4