code: function drawMaskingClip() {
              maskingRectangle = svg.selectAll('.chart-group')
                .append('clipPath')
                .attr('id', maskingRectangleId)
                  .append('rect')
                  .attr('width', chartWidth)
                  .attr('height', chartHeight)
                  .attr('x', 0)
                  .attr('y', 0);
          }
docs: Draws a masking clip for data points/circles
  to refer to. This will allow dots to have lower priority
  in the DOM.
  @return {void}
  @private