Vega-Lite: How to use slider value in transform calculation - vega

Question

How do I use the value of a slider in the transform?
This example from vega online editor plots a sine and cosine wave.
{
"$schema": "https://vega.github.io/schema/vega-lite/v4.json",
"description": "Plots two functions using a generated sequence.",
"width": 300,
"height": 150,
"data": {
"sequence": {
"start": 0,
"stop": 12.7,
"step": 0.1,
"as": "x"
}
},
"transform": [
{
"calculate": "sin(datum.x)",
"as": "sin(x)"
},
{
"calculate": "cos(datum.x)",
"as": "cos(x)"
},
{
"fold": ["sin(x)", "cos(x)"]
}
],
"mark": "line",
"encoding": {
"x": {
"type": "quantitative",
"field": "x"
},
"y": {
"field": "value",
"type": "quantitative"
},
"color": {
"field": "key",
"type": "nominal",
"title": null
}
}
}
I would like to add two sliders and use their values in the calculation. I can define sliders using:
"selection" : {
"amp" : {
"type" : "single",
"fields" : ["sin", "cos"],
"bind": {
"sin": { "input" : "range", "min": 0.0, "max": 10.0, "step": 0.1},
"cos": { "input" : "range", "min": 0.0, "max": 10.0, "step": 0.1}
}
}
},
How do I access the slider values to use in the calculations? Something like
{
"calculate": "amp.sin * sin(datum.x)",
"as": "sin(x)"
},

Answer 1

You can do this in exactly the way you wrote in your question. Additionally, adding an initial value will make the selections valid before you interact with them.
Here is a full example (vega editor):
{
"$schema": "https://vega.github.io/schema/vega-lite/v4.json",
"description": "Plots two functions using a generated sequence.",
"width": 300,
"height": 150,
"data": {"sequence": {"start": 0, "stop": 12.7, "step": 0.1, "as": "x"}},
"transform": [
{"calculate": "amp.sin * sin(datum.x)", "as": "sin(x)"},
{"calculate": "amp.cos * cos(datum.x)", "as": "cos(x)"},
{"fold": ["sin(x)", "cos(x)"]}
],
"mark": "line",
"encoding": {
"x": {"type": "quantitative", "field": "x"},
"y": {"field": "value", "type": "quantitative"},
"color": {"field": "key", "type": "nominal", "title": null}
},
"selection": {
"amp": {
"type": "single",
"fields": ["sin", "cos"],
"init": {"sin": 1, "cos": 1},
"bind": {
"sin": {"input": "range", "min": 0, "max": 10, "step": 0.1},
"cos": {"input": "range", "min": 0, "max": 10, "step": 0.1}
}
}
}
}