Cannot impute missing values - vega-lite

Question

Have this image
Given this vega-lite
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"data": {
"values": [
{
"timestamp": "2011-04-01T17:06:21.000Z",
"value": 0.44777528325189986
},
{
"timestamp": "2011-04-02T17:06:21.000Z",
"value": 0.44390285331388984
},
{
"timestamp": "2011-04-03T17:06:21.000Z",
"value": 0.44813958999449255
},
{
"timestamp": "2011-04-04T17:06:21.000Z",
"value": 0.4440416510172272
},
{
"timestamp": "2011-04-05T17:06:21.000Z",
"missing": "NO value KEY HERE!"
},
{
"timestamp": "2011-04-06T17:06:21.000Z",
"value": 0.3797480270068858
},
{
"timestamp": "2011-04-07T17:06:21.000Z",
"value": 0.31955288375970203
},
{
"timestamp": "2011-04-08T17:06:21.000Z",
"value": 0.3171368880067786
},
{
"timestamp": "2011-04-10T17:06:21.000Z",
"value": 0.30021395605134893
},
{
"timestamp": "2011-04-11T17:06:21.000Z",
"value": 0.3130485242947531
}
]
},
"encoding": {"y": {"field": "timestamp", "type": "temporal", "sort": "ascending"}},
"layer": [
{
"mark": {"type": "line", "interpolate": "cardinal"},
"encoding": {
"x": {
"field": "value",
"sort": null,
"type": "quantitative",
"axis": {"orient": "top"},
"impute": {"keyvals": ["value"], "method": "mean", "frame": [-5, 5]}
}
}
}
]
}
But I thought the impute line would cause it to fill that gap in the data:
"impute": {"keyvals": ["value"], "method": "mean", "frame": [-5, 5]}
Have tried many permutations of this, including:
changing keyvals to ["timestamp"]
Moving the impute line to inside the "encoding": {"y": ... definition
#2 but also switch keyvals to ["value"]
None of those seem to be working
Update
Also tried an impute in transform, and that doesn't work either:
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"data": {
"values": [
...
]
},
"transform": [
{
"impute": "value",
"key": "timestamp",
"frame": [-1, 1],
"method": "mean"
}
],
"encoding": {"y": {"field": "timestamp", "type": "temporal", "sort": "ascending"}},
"layer": [
{
"mark": {"type": "line", "interpolate": "cardinal"},
"encoding": {
"x": {
"field": "value",
"sort": null,
"type": "quantitative",
"axis": {"orient": "top"}
}
}
}
]
}
Update 2
Here's something that almost feels like progress, but doesn't behave how I would expect. This is the exact same data with the "transform" : [ "impute" : { ... approach, but now it's displaying imputed_value_value (which by the way is never mentioned in the docs) instead of value:
It does successfully impute, but it imputes (averages) everything, when I only want it to impute places with missing data. Is this how impute is supposed to work?
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"data": {
"values": [
...
]
},
"transform": [
{
"impute": "value",
"key": "timestamp",
"frame": [-5, 5],
"method": "mean"
}
],
"encoding": {"y": {"field": "timestamp", "type": "temporal", "sort": "ascending"}},
"layer": [
{
"mark": {"type": "line", "interpolate": "cardinal"},
"encoding": {
"x": {
"field": "imputed_value_value",
"sort": null,
"type": "quantitative",
"axis": {"orient": "top"},
}
}
}
]
}