dx *= (svg.viewBox.baseVal.width / swidth);
    dy *= (svg.viewBox.baseVal.height / sheight);

    svg.viewBox.baseVal.x -= dx;
    svg.viewBox.baseVal.y -= dy;
  }

  function handleScanStart(e) {
    if (e.button != 0) return; // Do not catch right-clicks etc.
    setMode(MOUSEPAN);
    panStart(e.clientX, e.clientY);
    e.preventDefault();
    svg.addEventListener('mousemove', handleScanMove);
  }