code: def vega_spec():
      return {
          "$schema": "https://vega.github.io/schema/vega/v5.json",
          "axes": [
              {
                  "aria": False,
                  "domain": False,
                  "grid": True,
                  "gridScale": "x",
                  "labels": False,
                  "maxExtent": 0,
                  "minExtent": 0,
                  "orient": "left",
                  "scale": "y",
                  "tickCount": {"signal": "ceil(height/40)"},
                  "ticks": False,
                  "zindex": 0,
              },
              {
                  "grid": False,
                  "labelAlign": "right",
                  "labelAngle": 270,
                  "labelBaseline": "middle",
                  "orient": "bottom",
                  "scale": "x",
                  "title": "a",
                  "zindex": 0,
              },
              {
                  "grid": False,
                  "labelOverlap": True,
                  "orient": "left",
                  "scale": "y",
                  "tickCount": {"signal": "ceil(height/40)"},
                  "title": "b",
                  "zindex": 0,
              },
          ],
          "background": "white",
          "data": [
              {
                  "name": "source_0",
                  "values": [
                      {"a": "A", "b": 28},
                      {"a": "B", "b": 55},
                      {"a": "C", "b": 43},
                      {"a": "D", "b": 91},
                      {"a": "E", "b": 81},
                      {"a": "F", "b": 53},
                      {"a": "G", "b": 19},
                      {"a": "H", "b": 87},
                      {"a": "I", "b": 52},
                  ],
              },
              {
                  "name": "data_0",
                  "source": "source_0",
                  "transform": [
                      {
                          "as": ["b_start", "b_end"],
                          "field": "b",
                          "groupby": ["a"],
                          "offset": "zero",
                          "sort": {"field": [], "order": []},
                          "type": "stack",
                      },
                      {
                          "expr": 'isValid(datum["b"]) && isFinite(+datum["b"])',
                          "type": "filter",
                      },
                  ],
              },
          ],
          "description": "A simple bar chart with embedded data.",
          "height": 200,
          "marks": [
              {
                  "encode": {
                      "update": {
                          "ariaRoleDescription": {"value": "bar"},
                          "description": {
                              "signal": '"a: " + (isValid(datum["a"]) ? datum["a"] : ""+datum["a"]) + "; b: " + (format(datum["b"], ""))'
                          },
                          "fill": {"value": "#4c78a8"},
                          "width": {
                              "signal": "max(0.25, bandwidth('x'))"
                          },
                          "x": {"field": "a", "scale": "x"},
                          "y": {"field": "b_end", "scale": "y"},
                          "y2": {"field": "b_start", "scale": "y"},
                      }
                  },
                  "from": {"data": "data_0"},
                  "name": "marks",
                  "style": ["bar"],
                  "type": "rect",
              }
          ],
          "padding": 5,
          "scales": [
              {
                  "domain": {
                      "data": "data_0",
                      "field": "a",
                      "sort": True,
                  },
                  "name": "x",
                  "paddingInner": 0.1,
                  "paddingOuter": 0.05,
                  "range": {"step": {"signal": "x_step"}},
                  "type": "band",
              },
              {
                  "domain": {
                      "data": "data_0",
                      "fields": ["b_start", "b_end"],
                  },
                  "name": "y",
                  "nice": True,
                  "range": [{"signal": "height"}, 0],
                  "type": "linear",
                  "zero": True,
              },
          ],
          "signals": [
              {"name": "x_step", "value": 20},
              {
                  "name": "width",
                  "update": "bandspace(domain('x').length, 0.1, 0.05) * x_step",
              },
          ],
          "style": "cell",
      }
  
file_path: altair/tests/utils/test_mimebundle.py
imports: import pytest
  import altair
  from altair import VEGA_VERSION
  from altair.utils.mimebundle import spec_to_mimebundle
label_desc: Use image node_id 2 for calling a global function with example usage: vega_spec() and returns: dict