That was changed in version 5, see Chapter Changes in version 5:
The reverse keyword of the set [axis]range command affects only autoscaling. It does not invert or otherwise alter the meaning of a command such as set xrange [0:1]. If you want to reverse the direction of the x axis in such a case, say instead set xrange [1:0].
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That was changed in version 5, see Chapter
Changes in version 5:See also Bug #1421