Building out the stat element of a layer

[ijlyttle]

In talking with @haleyjeppson, I think we see a way forward to deal with stats. Haley has already contributed some thoughts, in the Rmd file on the ggplot2 and Vega-Lite parts. I think it remains to sort out the ggspec part. I am proposing this issue as a gathering-place for that discussion.

I propose that Haley establish a branch to mirror this issue, so that we will merge a PR when we are "done". We can pr_pull_upstream(), as new things are merged into master from testing, ggschema, etc.

What follows is just a transcription from my notebook on a first idea for how a stat would appear in a ggspec layer; it is meant as a starting point for continuing the Rmd file.

This is meant to illustrate "new" parts of the spec, using stat-bin as an example:

{
  "layer": {
    "stat": {
      "class": "bin",
      "default_aes": {
        "y": {"stat_operation": "count"},
        "weight": {"value": 1}
      },
      "required_aes": ["x"]
    },
    "stat_params": {
      "binwidth": 10
    }
  }
}

The content of stat_params would depend on the class of the stat.

Also, I have modified some of this since talking with Haley, so I think we will have some discussions before "finalizing" anything.

[ijlyttle]

closed in #40