Gnuplot's documentation is downright baffling so it took me ages to work out if it was even possible to do a radar chart (aka spider chart, star plot, polar chart). Here's how I managed it in the end.
set angles degrees
set polar
set grid polar 120.
unset border
unset param
set style data filledcurves
set style fill solid 0.5
set datafile separator ","
set xtics axis nomirror
set ytics axis nomirror
set yrange [-45:45]
set xrange [-45:45]
set size square
set title "Radar chart"
plot '~/docs/mydata.csv' using 1:2 notitle, '' using 1:3 notitle, '' using 1:14 notitle, '' using 1:25 notitle
My datafile was a CSV file where the first column gave the angle in degrees (as you can see in the plot, I've only used three angles: 0, 120, 240). You can see in the last line of that code that I've separately chosen to plot polygons for columns 2, 3, 14, 25 from my data.