xAxis = {
const axis = d3.axisBottom()
.ticks(6)
.tickSize(size * columns.length);
return g => g.selectAll("g").data(x).join("g")
.attr("transform", (d, i) => `translate(${i * size},0)`)
.each(function(d) { return d3.select(this).call(axis.scale(d)); })
.call(g => g.select(".domain").remove())
.call(g => g.selectAll(".tick line").attr("stroke", "#ddd"));
}