prologues := 3;  u=18mm; textscale=u/12mm;  def disc =   begingroup;   save pmax,persp,persp_nt,tens,arr;   save rarrlft,rarrrt;   save p;   save axisprotrusion;   save xax,yax,zax;   pmax = 0;   persp_nt = 0.3;   tens = 2-persp_nt;   persp = persp_nt*u;   arr = 0.3u; % how far to offset the arrows from the diagram   axisprotrusion = 1u;  % how far do the axes protrude?   pair rarrlft,rarrrt; % endpoints for arrows   path p[]; % just plain paths for now   pair xax[],yax[],zax[]; % x,y,z are taken; these are for the axes   % solid paths   p0 = fullcircle scaled u xscaled 2 yscaled 2persp_nt;   % endpoints of dimension arrows   rarrlft = (0,0);   rarrrt = (1u,0);   % actually draw   pickup pencircle scaled 1pt;   for i=0 upto pmax:     draw p[i];   endfor;   drawdblarrow (rarrlft..rarrrt);   label.top(btex $r$ etex scaled textscale,.5[rarrlft,rarrrt]);   % x axis   xax1 = (1u,0);   xax2 = (1u+axisprotrusion*0.5,0);   drawarrow (xax1..xax2);   label.rt(btex $y$ etex scaled textscale,xax2);   % y axis   yax0 = (0,0);   yax1 = point 5 of p0;   yax2 = (.6+axisprotrusion/u)[yax0,yax1];   drawarrow (yax1..yax2);   draw yax0..yax1 dashed evenly;   label.ulft(btex $x$ etex scaled textscale,yax2);   % z axis   zax0 = (0,0);   zax1 = (0,axisprotrusion);   drawarrow (zax0..zax1);   label.top(btex $z$ etex scaled textscale,zax1);   endgroup; enddef;  beginfig(1)   disc;   currentpicture := currentpicture shifted (100,100); % avoid unwanted clipping endfig; end