code: function drawVerticalGridLines() {
              maskGridLines = svg.select('.grid-lines-group')
                  .selectAll('line.vertical-grid-line')
                  .data(xScale.ticks(xTicks))
                  .enter()
                   .append('line')
                    .attr('class', 'vertical-grid-line')
                    .attr('y1', (xAxisPadding.left))
                    .attr('y2', chartHeight)
                    .attr('x1', (d) => xScale(d))
                    .attr('x2', (d) => xScale(d));
          }
docs: Draws vertical gridlines of the chart
  These gridlines are parallel to y-axis
  @return {void}
  @private