code: function drawAxisLabels() {
              // If y-axis label is given, draw it
              if (yAxisLabel) {
                  if (yAxisLabelEl) {
                      svg.selectAll('.y-axis-label-text').remove();
                  }
  
                  yAxisLabelEl = svg.select('.axis-labels-group')
                      .append('g')
                      .attr('class', 'y-axis-label')
                        .append('text')
                        .classed('y-axis-label-text', true)
                        .attr('x', -chartHeight / 2)
                        .attr('y', yAxisLabelOffset - xAxisPadding.left)
                        .attr('text-anchor', 'middle')
                        .attr('transform', 'rotate(270 0 0)')
                        .text(yAxisLabel)
              }
  
              // If x-axis label is given, draw it
              if (xAxisLabel) {
                  if (xAxisLabelEl) {
                      svg.selectAll('.x-axis-label-text').remove();
                  }
  
                  xAxisLabelEl = svg.selectAll('.axis-labels-group')
                      .append('g')
                      .attr('class', 'x-axis-label')
                        .append('text')
                        .classed('x-axis-label-text', true)
                        .attr('x', chartWidth / 2)
                        .attr('y', chartHeight - xAxisLabelOffset)
                        .attr('text-anchor', 'middle')
                        .text(xAxisLabel);
              }
          }
docs: Draws axis labels next to x and y axis to
  represent data value labels on the chart
  @return {void}
  @private