//     x2 = u.x - (u.x - x1) * (w2 / w1)
    vb.baseVal.x = u.x - (u.x - vb.baseVal.x) * (w2 / w1);
    vb.baseVal.y = u.y - (u.y - vb.baseVal.y) * (h2 / h1);
  }

  function handleWheel(e) {
    if (e.deltaY == 0) return;
    // Change scale factor by 1.1 or 1/1.1
    rescale(currentScale * (e.deltaY < 0 ? 1.1 : (1/1.1)),
            toSvg(e.offsetX, e.offsetY));
  }

  function setMode(m) {