code: function drawArea(){
              if (area) {
                  svg.selectAll('.sparkline-area').remove();
              }
  
              area = d3Shape.area()
                  .x(({date}) => xScale(date))
                  .y0(() => yScale(0) + lineStrokeWidth / 2)
                  .y1(({value}) => yScale(value))
                  .curve(d3Shape.curveBasis);
  
              svg.select('.chart-group')
                .append('path')
                  .datum(data)
                  .attr('class', 'sparkline-area')
                  .attr('fill', `url(#${areaGradientId})`)
                  .attr('d', area)
                  .attr('clip-path', `url(#${maskingClipId})`);
          }
docs: Draws the area that will be placed below the line
  @private