code: function drawEndMarker(){
              if (circle) {
                  svg.selectAll('.sparkline-circle').remove();
              }
  
              circle = svg.selectAll('.chart-group')
                .append('circle')
                  .attr('class', 'sparkline-circle')
                  .attr('cx', xScale(data[data.length - 1].date))
                  .attr('cy', yScale(data[data.length - 1].value))
                  .attr('r', markerSize);
          }
docs: Draws a marker at the end of the sparkline