Truncate long tick labels with ellipses so they don’t get cropped.

[mbostock]

In a plot like this, it would be nice if the axis labels has something like text-overflow: ellipsis so that they don’t get cropped by the edge of the chart.

[mbostock]

With #1197 this generalizes into an option for the text mark, and given that the text mark already supports wrapping based on approximate text metrics, we could presumably generalize it to truncate instead of wrap using the same mechanism.