code: function drawSparklineTitle() {
              if (titleEl) {
                  svg.selectAll('.sparkline-text').remove();
              }
  
              titleEl = svg.selectAll('.text-group')
                .append('text')
                  .attr('x', chartWidth / 2)
                  .attr('y', chartHeight / 6)
                  .attr('text-anchor', 'middle')
                  .attr('class', 'sparkline-text')
                  .style('font-size', titleTextStyle['font-size'] || DEFAULT_TITLE_TEXT_STYLE['font-size'])
                  .style('fill', titleTextStyle['fill'] || lineGradient[0])
                  .style('font-family', titleTextStyle['font-family'] || DEFAULT_TITLE_TEXT_STYLE['font-family'])
                  .style('font-weight', titleTextStyle['font-weight'] || DEFAULT_TITLE_TEXT_STYLE['font-weight'])
                  .style('font-style', titleTextStyle['font-style'] || DEFAULT_TITLE_TEXT_STYLE['font-style'])
                  .text(titleText)
          }
docs: Draws the text element within the text group
  Is displayed at the top of sparked area
  @private