The set polar command switches gnuplot from its default
rectangular coordinate set to a polar coordinate set. In this mode,
the independant variable is the angle, and the dependent variable is
the radius. Here is an example of how to produce a circle (constant
radius of 1.58).
> plot("set polar");
> a = linspace(0,2*pi,100);
> b = zeros(100,1)+1.58;
> plot([a',b]);
> Warning: empty y range [1.58:1.58], adjusting to [1.422:1.738]
The warning message is from gnuplot, which wasn't expecting the range to
be constant, as that confuses the auto-scaling algorithm. The graph is
plotted as expected, gnuplot just pretends the range was larger.
A slightly more complex example is shown here:
> plot("set polar");
> a = linspace(0,2*pi,100);
> b = cos(2*a);
> plot([a',b']);
which produces a result of: