Ordering of a faceted Bar-chart in Vega - facet

Question

I have a question about facet visualizations in vega.
I have a pretty similar problem as the nested bar char example.
Here I changed the order of the tuple values. as intended, the order in the visualization is changing to.
But I want the 3 facets ordered ascending for value "a".
I don't really understand, if there is a "scale" for the facets, or where to order them.
Second part would be to have an axes for the value "a" shown below/above the bar chart.
Hope someone can help me.
Greetings
Christian
{
"$schema": "https://vega.github.io/schema/vega/v5.json",
"description": "A nested bar chart example, with bars grouped by category.",
"width": 300,
"padding": 5,
"autosize": "pad",
"signals": [
{
"name": "rangeStep", "value": 20,
"bind": {"input": "range", "min": 5, "max": 50, "step": 1}
},
{
"name": "innerPadding", "value": 0.1,
"bind": {"input": "range", "min": 0, "max": 0.7, "step": 0.01}
},
{
"name": "outerPadding", "value": 0.2,
"bind": {"input": "range", "min": 0, "max": 0.4, "step": 0.01}
},
{
"name": "height",
"update": "trellisExtent[1]"
}
],
"data": [
{
"name": "tuples",
"values": [
{"a": 1, "b": "b", "c": 4.4},
{"a": 0, "b": "c", "c": 5.1},
{"a": 0, "b": "a", "c": 6.3},
{"a": 0, "b": "a", "c": 4.2},
{"a": 0, "b": "b", "c": 6.8},
{"a": 2, "b": "b", "c": 3.5},
{"a": 2, "b": "c", "c": 6.2}
],
"transform": [
{
"type": "aggregate",
"groupby": ["a", "b"],
"fields": ["c"],
"ops": ["average"],
"as": ["c"]
}
]
},
{
"name": "trellis",
"source": "tuples",
"transform": [
{
"type": "aggregate",
"groupby": ["a"]
},
{
"type": "formula", "as": "span",
"expr": "rangeStep * bandspace(datum.count, innerPadding, outerPadding)"
},
{
"type": "stack",
"field": "span"
},
{
"type": "extent",
"field": "y1",
"signal": "trellisExtent"
}
]
}
],
"scales": [
{
"name": "xscale",
"domain": {"data": "tuples", "field": "c"},
"nice": true,
"zero": true,
"round": true,
"range": "width"
},
{
"name": "color",
"type": "ordinal",
"range": "category",
"domain": {"data": "trellis", "field": "a"}
}
],
"axes": [
{ "orient": "bottom", "scale": "xscale", "domain": true }
],
"marks": [
{
"type": "group",
"from": {
"data": "trellis",
"facet": {
"name": "faceted_tuples",
"data": "tuples",
"groupby": "a"
}
},
"encode": {
"enter": {
"x": {"value": 0},
"width": {"signal": "width"}
},
"update": {
"y": {"field": "y0"},
"y2": {"field": "y1"}
}
},
"scales": [
{
"name": "yscale",
"type": "band",
"paddingInner": {"signal": "innerPadding"},
"paddingOuter": {"signal": "outerPadding"},
"round": true,
"domain": {"data": "faceted_tuples", "field": "b"},
"range": {"step": {"signal": "rangeStep"}}
}
],
"axes": [
{ "orient": "left", "scale": "yscale",
"ticks": false, "domain": false, "labelPadding": 4 }
],
"marks": [
{
"type": "rect",
"from": {"data": "faceted_tuples"},
"encode": {
"enter": {
"x": {"value": 0},
"x2": {"scale": "xscale", "field": "c"},
"fill": {"scale": "color", "field": "a"},
"strokeWidth": {"value": 2}
},
"update": {
"y": {"scale": "yscale", "field": "b"},
"height": {"scale": "yscale", "band": 1},
"stroke": {"value": null},
"zindex": {"value": 0}
},
"hover": {
"stroke": {"value": "firebrick"},
"zindex": {"value": 1}
}
}
}
]
}
]
}