\documentclass[border=10pt]{standalone}
\usepackage{asymptote}
\begin{document}
\begin{asy}[width=\the\linewidth,inline=true]
settings.render=8;
settings.prc=false;
size(8cm);
import three;
import graph;
import fontsize; defaultpen(fontsize(9pt));
pen[] fillpen={
red, orange, yellow, green, lightblue, blue, darkblue
};
real
xmin=0, xmax=20,
ymin=0, ymax=20;
xaxis(xmin,xmax,RightTicks(Step=10,step=5));
yaxis(ymin,ymax, LeftTicks(Step=10,step=5));
real ra(real t, real a){return 3*a*sin(t)*cos(t)/(sin(t)^3+cos(t)^3);};
real r(real);
guide loop;
real a, a0=10, da=1;
int n=fillpen.length;
real t; pair p;
a=a0;
for(int i=0;i
Source: TeX.SE