Getting the last datum in a vega dataset - vega

Question

I have a data source A and I'd like to create a new data source B containing just the last element of A. What is the best way to do this in Vega?

Answer 1

This is relatively straight forward to do. Although I am slightly confused by your use of "max" in the aggregation since this isn't the last value?
Either way here is my solution for obtaining the last value in a dataset using this series of transforms,
transform: [
{
type: window
ops: [
row_number
]
}
{
type: joinaggregate
fields: [
row_number
]
ops: [
max
]
as: [
max_row_number
]
}
{
type: filter
expr: datum.row_number==datum.max_row_number
}
]

Answer 2

I was able to get this working in the Vega Editor using the following:
{
"$schema": "https://vega.github.io/schema/vega/v5.json",
"data": [
{
"name": "source",
"url": "https://raw.githubusercontent.com/vega/vega/master/docs/data/cars.json",
"transform": [
{
"type": "filter",
"expr": "datum['Horsepower'] != null && datum['Miles_per_Gallon'] != null && datum['Acceleration'] != null"
}
]
},
{
"name": "avg",
"source":"source",
"transform":[
{
"type":"aggregate",
"groupby":["Horsepower"],
"ops": ["average"],
"fields":["Miles_per_Gallon"],
"as":["Avg_Miles_per_Gallon"]
}
]
},
{
"name":"last",
"source": "avg",
"transform": [
{
"type": "aggregate",
"ops": ["max"],
"fields": ["Horsepower"],
"as": ["maxHorsepower"]
},
{
"type": "lookup",
"from": "avg",
"key": "Horsepower",
"fields": ["maxHorsepower"],
"values": ["Horsepower","Avg_Miles_per_Gallon"]
}
]
}
]
}
maxHorsepower
Horsepower
Avg_Miles_per_Gallon
230
230
16
I'd be interested to know if there are better ways, but this worked for me.