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Gnuplot version 4.0 deprecated certain syntax used in earlier versions, but continued to recognize it. Version 4.2 also accepts the deprecated syntax by default, but 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 future versions.
One major difference is the introduction of required keywords to disambiguate complicated commands. A notable issue was the use of bare numbers to specify offsets, line and point types. Illustrative examples:
Deprecated:
set data linespoints
plot <foo> 2 4
New:
set style data linespoints
plot <foo> linetype 2 pointtype 4
Another major compatibility issue is the interaction of the commands size and size. In earlier versions of gnuplot the behaviour of various terminal types with respect to size was very inconsistent. In the future, all terminals will behave the same way.
In version 4.2 most terminals follow the new convention (size has no effect on the output canvas size). However, the png/gif/jpeg terminal still follows the older, deprecated behaviour unless you have configured with `–disable-backwards-compatibility`. In the next version of gnuplot all terminal types will follow the new convention. You are strongly advised not to use size to control the size in pixels of png/gif/jpeg output files. Instead use `set term png size <XX>,<YY>`.
Please see size, size and the documentation for individual terminals.