timeUnit does not work after a flatten and flod transformation - transform

Question

Is it possible to use timeUnit after a flatten and flod transformation?
In the example below it doesnt work!
If I remove the timeUnit from the x axis it plots, but without the good things that come with the timeUnit.
Thanks
This is an example code that can be executed in the link below
https://vega.github.io/editor/#/edited
{
"$schema": "https://vega.github.io/schema/vega-lite/v4.json",
"description": "Sales in a Year.",
"width": 500,
"height": 200,
"data": {
"values": [
{"timestamp": ["2019-01-01","2019-02-01","2019-03-01","2019-04-01","2019-05-01","2019-06-01",
"2019-07-01","2019-08-01","2019-09-01","2019-10-01","2019-11-01","2019-12-01"],
"cars" : [55, 43, 91, 81, 53, 19, 87, 52, 52, 44, 52, 52],
"bikes" : [12, 6, 2, 0, 0, 0, 0, 0, 0, 3, 9, 15]}
]
},
"transform": [
{"flatten": ["timestamp", "cars", "bikes"]},
{"fold": ["cars", "bikes"]}
],
"mark": {"type":"bar", "tooltip": true, "cornerRadiusEnd": 4},
"encoding": {
"x": {"field": "timestamp",
"timeUnit": "month",
"type": "ordinal",
"title": "",
"axis": {"labelAngle": 0}},
"y": {"field": "value",
"type": "quantitative",
"title": "Soiling Loss"},
"color":{"field": "key",
"type": "nominal"}
}
}

Answer 1

For convenience, strings in input data with a simple temporal encoding are automatically parsed as dates, but such parsing is not applied to data that is the result of a transformation.
In this case, you can do the parsing manually with a calculate transform (view in editor):
{
"$schema": "https://vega.github.io/schema/vega-lite/v4.json",
"description": "Sales in a Year.",
"width": 500,
"height": 200,
"data": {
"values": [
{
"timestamp": [
"2019-01-01",
"2019-02-01",
"2019-03-01",
"2019-04-01",
"2019-05-01",
"2019-06-01",
"2019-07-01",
"2019-08-01",
"2019-09-01",
"2019-10-01",
"2019-11-01",
"2019-12-01"
],
"cars": [55, 43, 91, 81, 53, 19, 87, 52, 52, 44, 52, 52],
"bikes": [12, 6, 2, 0, 0, 0, 0, 0, 0, 3, 9, 15]
}
]
},
"transform": [
{"flatten": ["timestamp", "cars", "bikes"]},
{"fold": ["cars", "bikes"]},
{"calculate": "toDate(datum.timestamp)", "as": "timestamp"}
],
"mark": {"type": "bar", "tooltip": true, "cornerRadiusEnd": 4},
"encoding": {
"x": {
"field": "timestamp",
"timeUnit": "month",
"type": "ordinal",
"title": "",
"axis": {"labelAngle": 0}
},
"y": {"field": "value", "type": "quantitative", "title": "Soiling Loss"},
"color": {"field": "key", "type": "nominal"}
}
}