xAxis = g => g
.call(g => g.append("g")
.attr("transform", `translate(0,${margin.top})`)
.call(d3.axisTop(x).ticks(null, "d"))
.call(g => g.select(".domain").remove()))
.call(g => g.append("g")
.attr("transform", `translate(0,${innerHeight + margin.top + 4})`)
.call(d3.axisBottom(x)
.tickValues([data.year])
.tickFormat(x => x)
.tickSize(-innerHeight - 10))
.call(g => g.select(".tick text")
.clone()
.attr("dy", "2em")
.style("font-weight", "bold")
.text("Measles vaccine introduced"))
.call(g => g.select(".domain").remove()));