It's nice, but I think it's a bit more flexible to not have it as part of the
chart specification. This way, people can define titles however they like. It
might be nice to take a similar approach with reference ticks in the future.
We now preserve object constancy for ticks across transitions. By caching a
reference to the previous x-scale, we can initialize entering objects in the
correct location, then transition them to the new scale as they fade in. Also,
we use the `map` operator to convert the data to a standard representation that
is suitable for the bullet chart, and compute derivate data needed across
multiples.
Michael Bostock