let initScale = 1.0;     // currentScale when pinch zoom started
  let centerPoint = null;  // Center point for scaling

  // Convert event coordinates to svg coordinates.
  function toSvg(x, y) {
    const p = svg.createSVGPoint();
    p.x = x;
    p.y = y;
    let m = svg.getCTM();
    if (m == null) m = svg.getScreenCTM(); // Firefox workaround.
    return p.matrixTransform(m.inverse());
  }