drawHighlights(ctx) {
    this._highlights.forEach(giCiPi => {
      const [ gi, ci, pi ] = giCiPi;
      if (gi in this._glyphs && ci in this._glyphs[gi] 
        && pi in this._glyphs[gi][ci]) {
          const pt = this._glyphs[gi][ci][pi];
          const [ x2p, y2p ] = this._2p(gi);
          this.drawProjectPoint(ctx, x2p(pt.x), y2p(pt.y), 'highlight');
        }
    });
  }
Draw the highlighted points 
@param { CanvasRenderingContext2D } ctx