Adding a y-axis with absolute income

For this axis, we don't do anything special:

function addAxis(yIncomeScale, yIndexedScale, xScale, xRangeAdjusted) { 
    ... 
    var rightAxis = d3.axisRight().scale(yIncomeScale).ticks(20); 
    var rightAxisSVG = chart.append("g") 
        .attr('transform', 'translate( ' + (width + 4) + ')') 
        .call(rightAxis); 
    ... 
}

We just create d3.axisRight based on the yIncomeScale, and ask for twenty ticks. When we add this axis, we position the axis using the transform attribute and position it a little bit farther to the right to leave some room for the labels of the x-axis.

The axis on the left side will take some more work, since we're going to customize that a bit.