u=18mm;
 textscale=u/12mm;

 % this MetaPost script generates a PostScript file, which is then included in a
 % TeX document. if you want to generate EPS from this, you'll have to uncomment
 % the following line. (and probably do a couple more things, to boot.)
 %prologues := 1;

 def sphere =
   begingroup;
   save pmax,dmax,persp,persp_nt,tens,arr;
   save rarrlft,rarrrt;
   save p,d;
   save axisprotrusion;
   save xax,yax,zax;
   pmax = 0; dmax = 1;
   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 = 0.3u;  % how far do the axes protrude?
   pair rarrlft,rarrrt; % endpoints for arrows
   path p[],d[]; % just plain paths for now
   pair xax[],yax[],zax[]; % x,y,z are taken; these are for the axes
   % solid paths
   p0 = fullcircle scaled 2u;
   % dotted paths
   d0 = halfcircle scaled u xscaled 2 yscaled (2*persp_nt);
   d1 = halfcircle scaled u xscaled 2 yscaled (2*persp_nt) rotated 180;
   % endpoints of dimension arrows
   rarrlft = (0,0);
   rarrrt = (sqrt(0.5)*u,sqrt(0.5)*u);

   % actually draw
   pickup pencircle scaled 1pt;
   for i=0 upto pmax:
     draw p[i];
   endfor;
   for i=0 upto dmax:
     draw d[i] dashed evenly;
   endfor;
   drawdblarrow (rarrlft..rarrrt);
   label.top(btex $r$ etex scaled textscale,.5[rarrlft,rarrrt]);
   % x axis
   xax0 = (0,0);
   xax1 = (1u,0);
   xax2 = (1u+axisprotrusion,0);
   drawarrow (xax1..xax2);
   draw xax0..xax1 dashed evenly;
   label.rt(btex $x$ etex scaled textscale,xax2);
   % y axis
   yax0 = (0,0);
   yax1 = point 1 of d1;
   yax2 = (1.3+axisprotrusion/u)[yax0,yax1];
   drawarrow (yax1..yax2);
   draw yax0..yax1 dashed evenly;
   label.bot(btex $y$ etex scaled textscale,yax2);
   % z axis
   zax0 = (0,0);
   zax1 = (0,1u);
   zax2 = (0,1u+axisprotrusion);
   drawarrow (zax1..zax2);
   draw zax0..zax1 dashed evenly;
   label.top(btex $z$ etex scaled textscale,zax2);
   endgroup;
 enddef;

 beginfig(1)
   sphere;
 endfig;