Share y-scale and show only one axis in overlaid tracks

[sehilyi]

This happens when multiple tracks w/ different data are overlaid. This is not handled properly. Currently, only the tracks with the same data share the y axis.

Spec with an issue:

{
  "title": "HBM395.NRTL.659 Bulk ATAC-seq Data Raw",
  "subtitle": "Narrow Peaks and Summits",
  "views": [
    {
     "layout": "linear",
     "width": 800,
     "height": 180,
     "alignment": "overlay", 
     "tracks": [
      {
       "data": {
        "url": "https://gist.githubusercontent.com/ngehlenborg/9af4c34c65c9b820edda366a7af8d04d/raw/f8017ea25d9a735bb998e438763f2f993df5bde8/NA_peaks.narrowPeak",
        "type": "csv",
        "separator": "\t",
        "headerNames": ["chr", "start", "end", "name", "score", "strand", "signalValue", "pValue", "qValue", "peak" ],
        "chromosomeField": "chr",
        "genomicFields": ["start", "end"]

      },
      "mark": "bar",
      "x": {"field": "start", "type": "genomic"},
      "xe": {"field": "end", "type": "genomic"},
      "color":{ "value": "darkblue"},
      "stroke":{ "value": "darkblue"},
      "strokeWidth": {"value": 0.5},
      "y": {"field": "score", "type": "quantitative"}
    },
    {
     "data": {
      "url": "https://gist.githubusercontent.com/ngehlenborg/9af4c34c65c9b820edda366a7af8d04d/raw/f8017ea25d9a735bb998e438763f2f993df5bde8/NA_summits.bed",
      "type": "csv",
      "separator": "\t",
      "headerNames": ["chr", "start", "end", "name", "score"],
      "chromosomeField": "chr",
      "genomicFields": ["start", "end"]

    },
    "mark": "bar",
    "x": {"field": "start", "type": "genomic"},
    "xe": {"field": "end", "type": "genomic"},
    "stroke":{ "value": "lightblue"},
    "color":{ "value": "lightblue"},
    "strokeWidth": {"value": 1},
    "y": {"field": "score", "type": "quantitative"}
  }  
]
   }
 ]
}