set polar
unset border
unset tics
set rtics
set grid polar pi/4
set term pngcairo enh font "Arial,10" size 480,480
set out "polar_grid.png"
set size ratio -1
set rrange [0:1]
set samp 200
plot cos(2.5*t)**2 lw 2 lc rgb "dark-olivegreen"