Offset via encoding channels are centered at the ranges midpoint whereas offset in marks are centered at 0

[joelostblom]

If I set a random jitter via an encoding channel, the points will be centered in the band and jittered around the midpoint/tickmark as desired:

Open the Chart in the Vega Editor

    ...
    "yOffset": {"field": "random", "type": "quantitative"},
  ...
  "transform": [{"calculate": "random() * 15", "as": "random"}],

However, if I do the same thing in the mark config, the points are no longer jittered around a midpoint, but instead just in one direction ("down" in this screenshot), so that they are misaligned with the ticks on the y-axis:

Open the Chart in the Vega Editor

  "mark": {"type": "point", "yOffset": {"expr": "random() * 15"}},

This latter behavior is undesirable and it would be great if setting the offset in the mark config was centered just as when setting it via the encoding channel. Looking at the Vega spec, it seems that the mark uses a "point" scale whereas the encoding channel uses a "band" scale, but simple changing that did not fix the issue.

cc @kanitw since you implemented #7684 and might know best what is going on (I will tag you in a few offset related issues)