set polar
set border 0 lw 0          # raxis の線幅を 0 にする
unset tics
set term pngcairo enh font "Arial,10" size 480,480
set out "polar_noborder.png"
set size ratio -1
set samp 200
plot cos(2.5*t)**2 lw 2 lc rgb "sea-green"