code: function drawTick(y) {
      barGroup.appendChild(
        svgElement(
          legend,
          "line",
          {
            x1: legend.itemWidth,
            x2: legend.itemWidth + 5,
            y1: y,
            y2: y
          },
          "tick-mark"
        )
      );
    }
docs: draw a subtle tick to help indicate what the label refers to