function xGrid (g){
g
.attr('class', 'grid-lines')
.selectAll('line')
.data(xScale.ticks())
.join('line')
.attr('x1', d => xScale(d))
.attr('x2', d => xScale(d))
.attr('y1', margin.top)
.attr('y2', height - margin.bottom)
.style('stroke', "#E7E7E7")
}