contour_plot(sqrt(x^2+y^2),[x,-5,5],[y,-5,5], [legend,false], [gnuplot_preamble, "set cntrparam levels 10"]);

plot3d(sqrt(x^2+y^2),[x,-5,5],[y,-5,5],[legend, false], [gnuplot_preamble,"set view map;set contour;set cntrparam levels 30"]);