获取vega数据集中的最后一个数据

Question

我有一个数据源a，我想创建一个只包含a的最后一个元素的新数据源B。在Vega中，最好的方法是什么？

Answer 1

这是相对直接的做法。尽管我对你使用的"max"；因为这不是最后一个值？

无论哪种方式，这里都是我使用这一系列转换获得数据集中最后一个值的解决方案，

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

我能够使用以下工具在Vega编辑器中实现这一点：

{
"$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"]
}
]
}   
]
}

最大马力		平均里程数
230	330	16