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);
        }
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