reset set border set xtics 2 set ytics 2 set grid set xzeroaxis lt 9 lw 3 set yzeroaxis lt 9 lw 3 set style line 1 lt -1 lw 4 set style line 2 lt 5 lw 3 set style line 3 lt 12 lw 3 set xrange [-10:10] set yrange [-10:10] set samples 1000 set key off a=9 b=4 f(x)=sqrt((b**2)*(1-(x**2)/a**2)) g(x)=-sqrt((b**2)*(1-(x**2)/a**2)) plot f(x) ls 1, g(x) ls 1