Support tickvalues for axis

[ntucker]

If a values array is specified, the specified values are used for ticks, rather than using the scale's automatic tick generator. If values is null, clears any previously-set explicit tick values, reverting back to the scale's tick generator. If values is not specified, returns the currently-set tick values, which defaults to null. For example, to generate ticks at specific values:

var xAxis = d3.svg.axis()
.scale(x)
.tickValues([1, 2, 3, 5, 8, 13, 21]);

The explicit tick values take precedent over the tick arguments set by axis.ticks. However, any tick arguments will still be passed to the scale's tickFormat function if a tick format is not also set; thus, it may be valid to set both axis.ticks and axis.tickValues.

https://github.com/d3/d3/wiki/SVG-Axes#tickValues