code: function buildGradient() {
              if (!chartGradientEl) {
                  chartGradientEl = svg.select('.metadata-group')
                    .append('linearGradient')
                      .attr('id', gradientId)
                      .attr('gradientUnits', 'userSpaceOnUse')
                      .attr('x1', 0)
                      .attr('x2', xScale(data[data.length - 1].date))
                      .attr('y1', 0)
                      .attr('y2', 0)
                    .selectAll('stop')
                      .data([
                          {offset: '0%', color: gradient[0]},
                          {offset: '100%', color: gradient[1]}
                      ])
                    .enter().append('stop')
                      .attr('offset', ({offset}) => offset)
                      .attr('stop-color', ({color}) => color);
              }
          }
docs: Creates the gradient on the area
  @return {void}