set output "sinuses.mp"
set term mp color solid "csr10";
set grid;
set key left top;
set format x "$%0.2P\\pi$";
set xtics (-asin(90),-asin(90)/2, 0,asin(90)/2, asin(90))
set format y "$%1.1f$";
set xrange [-asin(180):asin(180)];
plot sin(x) title '$\sin(x)$', \
     sin(2*x) title '$\sin(2x)$', \
     cos(3*x) title '$\cos(3x)$';