yAxis (yAxis) {
        if (!arguments.length) {
            if (!this._yAxis) {
                this._yAxis = this._createYAxis();
            }
            return this._yAxis;
        }
        this._yAxis = yAxis;
        return this;
    }
Set or get the y axis used by the coordinate grid chart instance. This function is most useful
when y axis customization is required. Depending on `useRightYAxis` the y axis in dc.js is an instance of
either [d3.axisLeft](https://github.com/d3/d3-axis/blob/master/README.md#axisLeft) or
[d3.axisRight](https://github.com/d3/d3-axis/blob/master/README.md#axisRight); therefore it supports any
valid d3 axis manipulation.

**Caution**: The y axis is usually generated internally by dc; resetting it may cause
unexpected results.  Note also that when used as a getter, this function is not chainable: it
returns the axis, not the chart,
{@link https://github.com/dc-js/dc.js/wiki/FAQ#why-does-everything-break-after-a-call-to-xaxis-or-yaxis
so attempting to call chart functions after calling `.yAxis()` will fail}.
In addition, depending on whether you are going to use the axis on left or right
you need to appropriately pass [d3.axisLeft](https://github.com/d3/d3-axis/blob/master/README.md#axisLeft)
or [d3.axisRight](https://github.com/d3/d3-axis/blob/master/README.md#axisRight)
@see {@link https://github.com/d3/d3-axis/blob/master/README.md d3.axis}
@example
// customize y axis tick format
chart.yAxis().tickFormat(function(v) {return v + '%';});
// customize y axis tick values
chart.yAxis().tickValues([0, 100, 200, 300]);
@param {d3.axisLeft|d3.axisRight} [yAxis]
@returns {d3.axisLeft|d3.axisRight|CoordinateGridMixin}