[RFC] Support for interval (continuous) series

Nobody has claimed this yet.

In addition to supporting single-point Date drops (depicting a time when an event took place), I think support for interval (e.g. continuous events) would add some needed functionality to this component

I suppose the use cases range widely (and there are too many to count) but the most basic ones include:
- Scheduling
- Historical records / TImeline (membership / "activation time")
- Schematic view of recordings (e.g. go-pro / live feed / home cameras / etc)

I propose something of the sorts:

- A line/series is composed of either singular drops **or** intervals - specified as a boolean in the data object
- intervals are presented the same as drops, only 'circle' is replaced with a 'rect'

Open questions:
- Should meatball-ing appear the same for these two types?
- What is to be done with crossing intervals (unlike drops, they may contain dissimilar data)

Start by reviewing the existing single-point Date-drop data model and rendering behavior described in the issue. Define how intervals are represented, rendered as rectangles instead of circles, and handled when they cross or contain dissimilar data. Done requires decisions on interval data, crossing behavior, and meatball-ing before implementation can begin.

Written by the indexing model from the issue text.