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');
            }
        }
Draws an overlay element over the graph
@inner
@return void