function drawVerticalBars(layersSelection) {
            let layerJoin = layersSelection
                .data(layers);

            layerElements = layerJoin
                .enter()
                  .append('g')
                    .attr('fill', (({key}) => categoryColorMap[key]))
                    .classed('layer', true);

            let barJoin = layerElements
                    .selectAll('.bar')
                    .data((d) => filterOutUnkownValues(d));

            // Enter + Update
            let bars = barJoin
                    .enter()
                      .append('rect')
                        .classed('bar', true)
                        .attr('x', (d) => xScale(d.data.key))
                        .attr('y', (d) => yScale(d[1]))
                        .attr('width', xScale.bandwidth );

            if (isAnimated) {
                bars.style('opacity', barOpacity)
                    .transition()
                    .delay((_, i) => animationDelays[i])
                    .duration(animationDuration)
                    .ease(ease)
                    .tween('attr.height', verticalBarsTween);
            } else {
                bars.attr('height', (d) => yScale(d[0]) - yScale(d[1]));
            }
        }
Draws the bars along the y axis
@param  {D3Selection} layersSelection Selection of bars
@return {void}