Normally the margins of a plot are automatically calculated based on tics and axis labels (and the size of the graph correspondingly adjusted.) These computed values can be overridden by the set margin commands. show margin shows the current settings.
Syntax:
set bmargin {<margin>} set lmargin {<margin>} set rmargin {<margin>} set tmargin {<margin>} show margin
The units of <margin> are character heights or widths, as appropriate. A positive value defines the absolute size of the margin. A negative value (or none) causes gnuplot to revert to the computed value.