code: function drawHorizontalExtendedLine() {
              baseLine = svg.select('.axis-group')
                .selectAll('line.extended-x-line')
                .data([0])
                .enter()
                  .append('line')
                    .attr('class', 'extended-x-line')
                    .attr('x1', 0)
                    .attr('x2', chartWidth);
          }
docs: Draws a vertical line to extend x-axis till the edges
  @return {void}
  @private