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)
            }
        }
Builds the gradient element to be used later
@return {void}