code: function drawHoverOverlay(){
              if (!overlay) {
                  overlay = svg.select('.metadata-group')
                    .append('rect')
                      .attr('class','overlay')
                      .attr('y1', 0)
                      .attr('y2', height)
                      .attr('height', chartHeight)
                      .attr('width', chartWidth)
                      .attr('fill', overlayColor)
                      .style('display', 'none');
              }
          }
docs: Draws an overlay element over the graph
  @inner
  @return void