how to layer multiple regression lines without repeating code? - vega-lite

Question

I'm using Vega Lite to chart the times at which I eat my meals, and want a regression (loess) line showing the general time for each. By default, a regression uses the entire dataset and only shows one line; I want three lines, one for each meal (stored in the field extra_data.content).
I've achieved what I want to do my repeating the loess layer three times (screenshot) but am trying to find a solution in which the same layer is written once and repeats itself three times.
Edit after solving! Thanks very much to #jakevdp for the answer! Here is my working code; note that there is both a groupby on the loess and a color channel.
{
"mark": "line",
"transform": [
{
"loess": "hm",
"on": "ymd",
"groupby": ["extra_data.content"]
}
],
"encoding": {
"x": {
"field": "ymd",
"type": "temporal"
},
"y": {
"field": "hm",
"type": "temporal"
},
"color": {
"field": "extra_data.content",
"type": "nominal"
}
}
}

Answer 1

It sounds like you want the groupby argument of the loess transform, along with a color encoding. It might look something like this:
{
"mark": "line",
"transform": [
{
"loess": "hm",
"on": "ymd",
"groupby": ["extradata.content"]
}
],
"encoding": {
"x": {
"field": "ymd",
"type": "temporal"
},
"y": {
"field": "hm",
"type": "temporal"
},
"color": {
"field": "extradata.content",
"type": "nominal"
}
}
}