Add animated transitions to Vega.

Animated transition support is not planned for the initial 3.0 release, but would be nice to add in a subsequent minor release (e.g., 3.1). This issue is a migration of uwdata/vega#94.

Possible steps for implementing animated transitions in Vega 3:

• Design Vega 3 animation syntax (allow per-item duration/ease/delay within encode sets?)
• Add interpolation and scheduling support to vega-scenegraph
• Update Scale transform in vega-encode to persist previous scales
• Update vega-parser to allow access to previous scales
• Update Encoder in vega-encode to snapshot scenegraph item values prior to animation
• Update vega-view to orchestrate animation and retain animated exiting items (as needed)

[king612]

I was surprised actually that animations / transitions weren’t supported out of the box. This is a pretty standard feature of charting packages built on top of D3, and IMHO, one of the main value add’s of any framework built on top of D3 since update transitions can be pretty painful to implement. Please add!

[yuicer]

any process here?