|-------|-------|-------|-------|-------|-------|
 nov     dec     jan     feb     mar     apr     may

|-------|-------|-------|-------|-------|-------|
     nov     dec     jan     feb     mar     apr

// scale is your usual x scale. change dates on the domain by 15 days
var d0 = scale.domain()[0];
d0.setDate(d0.getDate() - 15);
var d1 = scale.domain()[1];
d1.setDate(d1.getDate() - 15);

// use this new x scale for the axis only
new_scale = d3.time.scale().domain([d0, d1]).range(scale.range());
d3.svg.axis().scale(new_scale);

// ticksDistance is constant for a specific x_scale
const getTicksDistance = (scale, ticks) => {
      const tcks = scale.ticks(ticks);
      const spaces = []
      for(let i=0; i < tcks.length - 1; i++){
        spaces.push(scale(tcks[i+1]) - scale(tcks[i]))
      }
      return spaces;
};
const ticksSpacingTime = getTicksDistance(x_scale_time, d3.timeMonth.every(1));

svg.append("g")
    .attr("class", "x-axis-shifted")
    .attr("transform", "translate(0,100)")
    .call(x_axis)
  .selectAll("text")
    .attr("x", (d,i) => ticksSpacingTime[i]/2)