How to Annotate a Line in line Chart in Vega-Lite? - vega-lite

Question

How to annotate a line in line chart in vega-lite
For the below code https://vega.github.io/editor/#/
{
"$schema": "https://vega.github.io/schema/vega-lite/v5.json",
"layer": [
{
"data": {"url": "data/stocks.csv"},
"mark": "line",
"encoding": {
"x": {"field": "date", "type": "temporal"},
"y": {"field": "price", "type": "quantitative"},
"color": {"field": "symbol", "type": "nominal"}
}
},
{
"data": {"values": [{}]},
"mark": {"type": "rule", "strokeDash": [2, 2], "size": 2},
"encoding": {"x": {"datum": {"year": 2006}}}
}
]
}
We get plot
If I want to annotate the line at a specific position like (2004,400)
I tried this, it is working, but I don't want to pass hardcoded values like "a": 2004, "b": 400,
{
"data": {
"values": [
{"a": 2004, "b": 400}
]
},
"mark": {"type": "text", "fontSize" : 16, "fontWeight":"bold", "align" : "left"},
"encoding": {
"text": {"value": "Optimum"},
"x": {"field": "a", "type": "quantitative", "title":""},
"y": {"field": "b", "type": "quantitative", "title":""}
}
},
How to pass specific values from the data like average value of date (say:2004) and average value of price (say:400)?
or
just next to the line in the middle of y-axis

Answer 1

Transform and Aggregate Worked, I needed only for Y axis average position, so below code worked good. For another axis we can use same transform.
{
"mark": {"type": "text", "fontSize" : 16, "fontWeight":"bold", "align" : "left"},
"transform": [
{
"aggregate": [{
"op": "mean",
"field": "price",
"as": "mean_y_axis"
}],
"groupby": ["date"]
}
],
"encoding": {
"text": {"value": "Optimum"},
"x": {"field": "date",
"type": "quantitative"},
"y": {"field": "mean_y_axis",
"type": "quantitative"}
}
}