function drawHoverOverlay() {
            // Not ideal, we need to figure out how to call exit for nested elements
            if (overlay) {
                svg.selectAll('.overlay').remove();
            }

            overlay = svg.select('.metadata-group')
              .append('rect')
                .attr('class', 'overlay')
                .attr('y1', 0)
                .attr('y2', chartHeight)
                .attr('height', chartHeight)
                .attr('width', chartWidth)
                .attr('fill', overlayColor)
                .style('display', 'none');
        }
Draws an overlay element over the graph
@private