examples/plot_3d_surface/plot.rb
require 'gnuplotrb'
include GnuplotRB
double_pi = Math::PI * 2
plot3d = Splot.new('cos(x)*cos(y)', xrange: -double_pi..double_pi, yrange: -double_pi..double_pi, style: 'function lp', hidden3d: true)
plot3d.to_png('./gnuplot_gem.png', size: [600, 600])