Previously we were using string coercion as the key function for axis ticks.
However, when the stringified value of the tick does not fully capture the
representation (such as a date with millisecond precision, whose string form
only has second precision; fixes #1529), string coercion is insufficient.

Fortunately, there is an equivalently-simple key function for tick identity: the
scale! If the scale does not return a unique position for the given tick, then
the tick would be overlapping, so it serves perfectly as the key function.