в gnuplot квадратная сетка

Рисовать окружности ... хм ... может тебе нужен графический редактор, а не gnuplot?

http://scs.ictp.it/manuals/gnuplot/gnuplot_165.html :
<quote>
set size {{no}square | ratio <r> | noratio} {<xscale>,<yscale>}
show size

The <xscale> and <yscale> values are the scaling factors for the size of the plot, which includes the graph and the margins.
`ratio` causes `gnuplot` to try to create a graph with an aspect ratio of <r> (the ratio of the y-axis length to the x-axis length) within the portion of the plot specified by <xscale> and <yscale>.
The meaning of a negative value for <r> is different. If <r>=-1, gnuplot tries to set the scales so that the unit has the same length on both the x and y axes (suitable for geographical data, for instance). If <r>=-2, the unit on y has twice the length of the unit on x, and so on.
</quote>

Нажми на клавишу "7".