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.