input.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "autosize": "pad",
  "padding": 5,
  "height": 300,
  "style": "cell",
  "data": [
    {
      "name": "938224664827150147",
      "transform": [{"type": "queryibis", "name": "938224664827150147"}]
    },
    {
      "name": "data_0",
      "source": "938224664827150147",
      "transform": [
        {
          "type": "aggregate",
          "groupby": ["c"],
          "ops": ["mean"],
          "fields": ["b"],
          "as": ["mean_b"]
        }
      ]
    }
  ],
  "signals": [
    {"name": "x_step", "value": 20},
    {
      "name": "width",
      "update": "bandspace(domain('x').length, 0.1, 0.05) * x_step"
    }
  ],
  "marks": [
    {
      "name": "marks",
      "type": "rect",
      "style": ["bar"],
      "from": {"data": "data_0"},
      "encode": {
        "update": {
          "fill": [
            {
              "test": "datum[\"mean_b\"] === null || isNaN(datum[\"mean_b\"])",
              "value": null
            },
            {"value": "#4c78a8"}
          ],
          "tooltip": {
            "signal": "{\"c\": ''+datum[\"c\"], \"Mean of b\": format(datum[\"mean_b\"], \"\")}"
          },
          "x": {"scale": "x", "field": "c"},
          "width": {"scale": "x", "band": true},
          "y": {"scale": "y", "field": "mean_b"},
          "y2": {"scale": "y", "value": 0}
        }
      }
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "data_0", "field": "c", "sort": true},
      "range": {"step": {"signal": "x_step"}},
      "paddingInner": 0.1,
      "paddingOuter": 0.05
    },
    {
      "name": "y",
      "type": "linear",
      "domain": {"data": "data_0", "field": "mean_b"},
      "range": [{"signal": "height"}, 0],
      "nice": true,
      "zero": true
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "c",
      "labelAlign": "right",
      "labelAngle": 270,
      "labelBaseline": "middle",
      "zindex": 1
    },
    {
      "scale": "y",
      "orient": "left",
      "grid": false,
      "title": "Mean of b",
      "labelOverlap": true,
      "tickCount": {"signal": "ceil(height/40)"},
      "zindex": 1
    },
    {
      "scale": "y",
      "orient": "left",
      "gridScale": "x",
      "grid": true,
      "tickCount": {"signal": "ceil(height/40)"},
      "domain": false,
      "labels": false,
      "maxExtent": 0,
      "minExtent": 0,
      "ticks": false,
      "zindex": 0
    }
  ]
}