I want to pick the previous value when my current value is undefined. The below code works fine when only 1 value is missing. But when multiple consecutive values are missing in my dataset, I cannot replace them with the any previous value that is available. For example, in the below dataset, the value of y is missing when x=3 and c=0. So it picks the data from x=2 and c=0 and replaces the missing value with y=51. But when multiple consecutive values of y is missing for c=0 and x=6, 7 and 8, only the value of y for x=6 is being replaced by 24, i.e., the value of y when x=5. I want y to be assigned 24 for x=7 and 8 as well. Any help would be appreciated. Thanks in advance!

```
{
"$schema": "https://vega.github.io/schema/vega/v5.json",
"width": 500,
"height": 200,
"padding": 5,
"signals": [
{
"name": "undefined",
"value": undefined
}
],
"data": [
{
"name": "table",
"values": [
{"x": 0, "y": 28, "c":0}, {"x": 0, "y": 20, "c":1},
{"x": 1, "y": 43, "c":0}, {"x": 1, "y": 35, "c":1},
{"x": 2, "y": 51, "c":0}, {"x": 2, "y": 10, "c":1},
{"x": 3, "c":0}, {"x": 3, "y": 15, "c":1},
{"x": 4, "y": 52, "c":0}, {"x": 4, "y": 48, "c":1},
{"x": 5, "y": 24, "c":0}, {"x": 5, "y": 28, "c":1},
{"x": 6, "c":0}, {"x": 6, "y": 66, "c":1},
{"x": 7, "c":0}, {"x": 7, "y": 27, "c":1},
{"x": 8, "c":0}, {"x": 8, "y": 16, "c":1},
{"x": 9, "y": 49, "c":0}, {"x": 9, "y": 25, "c":1}
],"transform": [{"type": "window", "frame": [
1,
0
], "groupby": ["c"], "fields": ["y"], "ops":["first_value"]
}, {"type": "formula", "expr": "if(datum['y']===datum[undefined],datum['first_value_y'],datum['y'])", "as": "y"}
]}],
"scales": [
{
"name": "x",
"type": "point",
"range": "width",
"domain": {"data": "table", "field": "x"}
},
{
"name": "y",
"type": "linear",
"range": "height",
"nice": true,
"zero": true,
"domain": {"data": "table", "field": "y"}
},
{
"name": "color",
"type": "ordinal",
"range": "category",
"domain": {"data": "table", "field": "c"}
}
],
"axes": [
{"orient": "bottom", "scale": "x"},
{"orient": "left", "scale": "y"}
],
"marks": [
{
"type": "group",
"from": {
"facet": {
"name": "series",
"data": "table",
"groupby": "c"
}
},
"marks": [
{
"type": "line",
"from": {"data": "series"},
"encode": {
"enter": {
"x": {"scale": "x", "field": "x"},
"y": {"scale": "y", "field": "y"},
"stroke": {"scale": "color", "field": "c"},
"strokeWidth": {"value": 2}
},
"update": {
"fillOpacity": {"value": 1}
},
"hover": {
"fillOpacity": {"value": 0.5}
}
}
},
{
"type": "symbol",
"from": {"data": "series"},
"encode": {
"enter": {
"x": {"scale": "x", "field": "x"},
"y": {"scale": "y", "field": "y"},
"fill": {"scale": "color", "field": "c"}
},
"update": {
"fillOpacity": {"value": 1}
},
"hover": {
"fillOpacity": {"value": 0.5}
}
}
}
]
}
]
}
```