Impute transform with frame parameter

[qnzhou]

This is an followup from the discussion of this Atlair discussion thread: vega/altair#2947

In summary, there seems to be a mismatch between the documentation and the results we are getting from Altair/vega-lite. Here is a minimum example to reproduce the problem (credit: @ChristopherDavisUCI).

The x variable is missing the value corresponding to t=3 and the y variable is missing the value corresponding to t=1. With "frame": [-1, 1] and "method": "mean", we are expecting x=4.5 at t=3 and y=2.5 at t=1. However, this is not the case as shown in the result plot (which indicates x=4 and y=3). Any insight as to why this is the case will be very helpful!

[domoritz]

Is this an issue with Vega-Lite
or Vega?

[qnzhou]

Could be, but Vega's impute documentation never mentions the support of a frame parameter. On the other hand, frame is on the official Vega-Lite doc...