In bar charts, we need to create a second scale that will inform us how wide each bar series should be, and where they should be placed. For this, we use postUpdateScale to calculate a second ordinal scale, with reference to the first:

function postUpdateScale(self, scaleData, mainData, compData) {
  self.xScale2 = d3.scale.ordinal()
    .domain(d3.range(0, mainData.length))
    .rangeRoundBands([0, self.xScale.rangeBand()], 0.08);
}