code: function buildGradient() {
              if (!linearGradient) {
                  linearGradient = svg.select('.metadata-group')
                    .append('linearGradient')
                      .attr('id', lineGradientId)
                      .attr('x1', '0%')
                      .attr('y1', '0%')
                      .attr('x2', '100%')
                      .attr('y2', '0%')
                      .attr('gradientUnits', 'userSpaceOnUse')
                      .selectAll('stop')
                      .data([
                          {offset:'0%', color: singleLineGradientColors[0]},
                          {offset:'100%', color: singleLineGradientColors[1]}
                      ])
                      .enter()
                        .append('stop')
                          .attr('offset', ({offset}) => offset)
                          .attr('stop-color', ({color}) => color)
              }
          }
docs: Builds the gradient element to be used later
  @return {void}