How to specify Rule Line with a single value in VegaLite? - vega

Question

It's possible to specify Rule as encoding over the data. But sometimes it's too verbose. Is there a shorter way to specify it with just a single number?
In example below I want to draw a horizontal line with y=1 and it requires me to specify a calculate transform. Wonder if it's possible with something more compact, like:
"layer": [
...,
{ "mark": { "type": "rule", "y": 1 }, // Specify ruler with just a single line
...
]
Playground
{
"$schema": "https://vega.github.io/schema/vega-lite/v4.json",
"transform":[
{ "calculate": "1", "as": "one" }
],
"layer": [
{
"mark": { "type": "bar" },
"encoding": {
"x": { "field": "diff", "type": "quantitative" },
"y": { "field": "diff", "type": "quantitative" }
}
},
{
"mark": { "type": "rule" },
"encoding": {
"y": { "field": "one", "type": "quantitative" }
}
}
],
"data": {
"values": [
{ "diff": 1 },
{ "diff": 2 },
{ "diff": 3 }
]
}
}

Answer 1

You can specify a single value using the "value" field in the encoding; e.g.
{
"mark": { "type": "rule" },
"encoding": {
"y": { "value": 133 }
}
}
(Playground)
But be aware this value is a value in the axis domain, rather than in the data domain; i.e. it represents pixels from the top-left of the chart.
If you want to specify a value in the data domain, there is no such shorthand. You have to either use a transform (as you did in your question) or define a new dataset for your layer. The most compact way of doing this, I think, is something like this:
{
"mark": { "type": "rule" },
"data": {"values": {"y": 1}},
"encoding": {
"y": { "field": "y" }
}
}