prologues := 3;

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

def cone =
  begingroup;
  save pmax,dmax,persp,persp_nt,tens,arr;
  save harr,rarr;
  save p,d;
  save axisprotrusion;
  save xax,yax,zax;
  save height,width,heightu,widthu;
  pmax = 2; dmax = 0;
  persp_nt = 0.5;
  tens = 2-persp_nt;
  persp = persp_nt*u;
  arr = 0.3u; % how far to offset the arrows from the diagram
  axisprotrusion = 0.4u;  % how far do the axes protrude?
  pair harr[],rarr[]; % 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
  % parameters for the actual cone
  height = 1;
  width = 1.5;
  heightu = height*u;
  widthu = width*u;
  % solid paths
  p0 = (0,0)--(widthu/2,heightu);
  p1 = (0,0)--(-widthu/2,heightu);
  p2 = fullcircle scaled u xscaled width yscaled persp_nt shifted (0,heightu);
  % dotted paths
  % don't draw this; it's reference for the y-axis
  d99 = halfcircle scaled u xscaled 2 yscaled (2*persp_nt) rotated 180;
  % endpoints of dimension arrows
  harr0 = (widthu/2,0);
  harr1 = (widthu/2,heightu);
  rarr0 = (0,heightu);
  rarr1 = (widthu/2,heightu);

  % actually draw
  pickup pencircle scaled 1pt;
  for i=0 upto pmax:
    draw p[i];
  endfor;
  drawdblarrow (harr0..harr1);
  label.rt(btex $h$ etex scaled textscale,.5[harr0,harr1]);
  drawdblarrow (rarr0..rarr1);
  label.top(btex $r$ etex scaled textscale,.3[rarr0,rarr1]);
  % x axis
  xax0 = (0,0);
  xax1 = (widthu/2+arr,0);
  drawarrow (xax0..xax1);
  label.rt(btex $x$ etex scaled textscale,xax1);
  % y axis
  yax0 = (0,0);
  yax1 = point 1 of d99;
  yax2 = (width*persp_nt)[yax0,yax1];
  drawarrow (yax0..yax2);
  label.bot(btex $y$ etex scaled textscale,yax2);
  % z axis
  zax0 = (0,0);
  zax1 = (0,heightu);
  zax2 = (0,heightu+axisprotrusion);
  drawarrow (zax1..zax2);
  draw zax0..zax1 dashed evenly;
  label.top(btex $z$ etex scaled textscale,zax2);
  endgroup;
enddef;

beginfig(1)
  cone;
  currentpicture := currentpicture shifted (100,100); % avoid unwanted clipping
endfig;
end