define mechanism to control which tick values appear on an axis

[heckj]

Axis has requestedTickLabels, with an array of values, but today that's not used.

• Wire that through so that - if provided - these values are used to drive the specific tick labels and locations
• rework Axis and the .xAxis() and .yAxis() modifiers to default to nil for those values, and use that differently from an empty list, the empty list being "don't show any ticks or labels"