ENH: logviewer scroll charts, custom data transforms

[pratapvardhan]

• ENH: logviewer vega charts to scroll fit

If you want to see 100 items of bars, and scrolled within 250px view, this enables it.

for vegam:
 [ ...,
  {apply: 'update', autosize: {"contains": "padding", "type": "fit-x"}},
   ...]

---

• ENH: logviewer transform support custom function

Supports custom python functions to transform data

transforms:
      - type: function
        expr: custom.custom(data)

[sanand0]

@pratapvardhan -- this is all logviewer related. I'll merge tomorrow without review :-)