Preserving object constancy across transitions is tricky! For example, what
happens if we remove the whiskers in a transition? How do we join outliers? This
commit makes a few assumptions explicit:
1. The `quartiles` function must return exactly three elements. This property
must be specified as a function.
2. The `whiskers` function must return exactly 2 elements, or null if no
whiskers are to be displayed. This property must be specified as a function.
3. The `domain` function must return exactly 2 elements, or null if the default
domain should be used. This property can be specified either as a constant or as
a function.
We could generalize this chart to support more than two whiskers, but it doesn't
seem urgent, and it would complicate the transition if the number of whiskers
changes. In a related change, the `whiskers` function does not receive a third
argument containing the quartiles; instead, this is made available by the
`quartiles` property on the values array (the first argument).
The outliers are joined using the `Number` key function. The outlier data is now
stored as indices; this allows reasonable object constancy across transitions
with outliers. Similarly, the tick labels for the quartiles are whiskers are now
separated, such that the whisker labels can be added or removed without spurious
transition.
This example shows how to preserve scales across multiples using a single chart
instance, as opposed to bullet-multiples which uses distinct chart instances to
supply separate scales. I still think it'd be better to use a single chart
instance in both cases, but then I need a different place to hide the scale
state.
This way, we get separate scales for the small multiples, which makes sense
given our data. However, I'm not totally convinced this is the right way to
implement separate scales, because it's a bit awkward to create separate chart
instances that look identical. Also, it's unfortunate that the charts are
stateful; it'd be better to somehow store the scale as data on the nodes, so
that chart specifications could be more easily reused. Then, there might be a
method to fix the domain rather than computing the domain per-chart.
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.
Mike Bostock
Jason Davies