code: function drawLines(){
              let lines,
                  topicLine;
  
              topicLine = d3Shape.line()
                  .curve(curveMap[lineCurve])
                  .x(({date}) => xScale(date))
                  .y(({value}) => yScale(value));
  
              lines = svg.select('.chart-group').selectAll('.line')
                  .data(dataByTopic, getTopic);
  
              paths = lines.enter()
                .append('g')
                  .attr('class', 'topic')
                .append('path')
                  .attr('class', 'line')
                  .merge(lines)
                  .attr('id', ({topic}) => topic)
                  .attr('d', ({dates}) => topicLine(dates))
                  .style('stroke', (d) => (
                      dataByTopic.length === 1 ? `url(#${lineGradientId})` : getLineColor(d)
                  ));
  
              lines
                  .exit()
                  .remove();
          }
docs: Draws the line elements within the chart group
  @private