prologues := 3;

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

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 $y$ 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 $x$ 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;
  currentpicture := currentpicture shifted (100,100); % avoid unwanted clipping
endfig;
end