canvasDiv.className = "tl-canvas";
    canvasDiv.style.position = "absolute";
    parent.appendChild(canvasDiv);
  
    var elems = {};
    var elemClasses = [
      "rail", "mergeoffset", "node", "arrow u", "arrow u sm", "line",
      "arrow merge r", "line merge", "arrow warp", "line warp",
      "line cherrypick"
    ];
    for( var i=0; i<elemClasses.length; i++ ){
      var cls = elemClasses[i];
      var elem = document.createElement("div");
      elem.className = "tl-" + cls;
      if( cls.indexOf("line")==0 ) elem.className += " v";
      canvasDiv.appendChild(elem);
      var k = cls.replace(/\s/g, "_");
      var r = elem.getBoundingClientRect();
      var w = Math.round(r.right - r.left);
      var h = Math.round(r.bottom - r.top);
      elems[k] = {w: w, h: h, cls: cls};
    }
    node = elems.node;
    arrow = elems.arrow_u;
    arrowSmall = elems.arrow_u_sm;
    line = elems.line;
    mArrow = elems.arrow_merge_r;
    mLine = elems.line_merge;
    cpLine = elems.line_cherrypick;
    wArrow = elems.arrow_warp;
    wLine = elems.line_warp;
  
    var minRailPitch = Math.ceil((node.w+line.w)/2 + mArrow.w + 1);
    if( window.innerWidth<400 ){
      railPitch = minRailPitch;
    }else{

    var y0 = from.y + node.h/2;
    var y1 = Math.ceil(to.y + node.h + arw.h/2);
    drawLine(line,color,x,y0,null,y1);
    x = to.x + (node.w-arw.w)/2;
    var n = drawBox(arw.cls,null,x,y);
    if(color) n.style.borderBottomColor = color;
  }
  /* Draw thin horizontal or vertical lines representing merges */
  function drawMergeLine(x0,y0,x1,y1){
    drawLine(mLine,null,x0,y0,x1,y1);
  }
  function drawCherrypickLine(x0,y0,x1,y1){
    drawLine(cpLine,null,x0,y0,x1,y1);
  }
  /* Draw an arrow representing an in-bound merge from the "rail"-th rail
  ** over to the node of "p".  Make is a checkpoint merge is "isCP" is true */
  function drawMergeArrow(p,rail,isCP){
    var x0 = rail*railPitch + node.w/2;
    if( rail in mergeLines ){
      x0 += mergeLines[rail];
      if( p.r<rail ) x0 += mLine.w;
    }else{
      x0 += (p.r<rail ? -1 : 1)*line.w/2;
    }
    var x1 = mArrow.w ? mArrow.w/2 : -node.w/2;
    x1 = p.x + (p.r<rail ? node.w + Math.ceil(x1) : -x1);
    var y = miLineY(p);

    var x = p.x + (p.r<rail ? node.w : -mArrow.w);
    var cls;
    if( isCP ){
      drawCherrypickLine(x0,y,x1,null);
      cls = "arrow cherrypick " + (p.r<rail ? "l" : "r");
    }else{
      drawMergeLine(x0,y,x1,null);
      cls = "arrow merge " + (p.r<rail ? "l" : "r");
    }
    drawBox(cls,null,x,y+(mLine.w-mArrow.h)/2);
  }
  function drawNode(p, btm){
    if( p.bg ){
      var e = document.getElementById("mc"+p.id);
      if(e) e.style.backgroundColor = p.bg;
      e = document.getElementById("md"+p.id);

    n.id = "tln"+p.id;
    n.onclick = clickOnNode;
    n.style.zIndex = 10;
    if( !tx.omitDescenders ){
      if( p.u==0 ) drawUpArrow(p,{x: p.x, y: -node.h},p.fg);
      if( p.hasOwnProperty('d') ) drawUpArrow({x: p.x, y: btm-node.h/2},p,p.fg);
    }
    if( p.hasOwnProperty('mo') ){
      var x0 = p.x + node.w/2;
      var x1 = p.mo*railPitch + node.w/2;
      var u = tx.rowinfo[p.mu-tx.iTopRow];
      var y1 = miLineY(u);
      if( p.u<0 || p.mo!=p.r ){
        x1 += mergeLines[p.mo] = -mLine.w/2;
        var y0 = p.y+2;

        var rail = p.mi[i];
        if( rail<0 ){
          rail = -rail;
          mergeLines[rail] = -mLine.w/2;
          var x = rail*railPitch + (node.w-mLine.w)/2;
          drawMergeLine(x,miLineY(p),null,btm);
        }
        drawMergeArrow(p,rail,0);
      }
    }
    if( p.hasOwnProperty('cpi') ){
      for( var i=0; i<p.cpi.length; i++ ){
        var rail = p.cpi[i];
        if( rail<0 ){
          rail = -rail;
          mergeLines[rail] = -mLine.w/2;
          var x = rail*railPitch + (node.w-mLine.w)/2;
          drawMergeLine(x,miLineY(p),null,btm);
        }
        drawMergeArrow(p,rail,1);
      }
    }
  }
  var mergeLines;
  function renderGraph(){
    mergeLines = {};
    canvasDiv.innerHTML = "";