Summarize transform not working, but doesn't throw error

[1wiseash]

Hi,
Great project and very useful. Unfortunately, I have hit a road block. I can't seem to get summarize transforms to work. I have experimented with all manner of formats for the specification to make sure it isn't a simple user error (or try to convince myself of that anyway). The final spec (listed below) doesn't throw any errors, but doesn't work either.
I am basically trying to find the earliest time in each feature of a geojson file, then offset the other time fields using the feature's base time (converting to minutes also). Using a fixed value for base_time works to compute all the fields works fine ({"as": "base_time", "calculate": "1483228800000"}), so I am sure the subsequent calculations are encoded correctly. But when I try to use the summarize transform, all my other fields break, including fields that don't refer to base_time. The specification I have is below (leaving out the data, as it is very large).
Thanks in advance for your help.
-Ashley
{
"data": {...},
"transform": [
{
"filter": "datum.time > 0"
},
{
"filter": "datum.eta > 0"
},
{
"filter": "datum.sta > 0"
},
{
"summarize": [
{
"aggregate": "min",
"field": "time",
"as": "base_time"
}
],
"groupby": [
"feature_id"
]
},
{
"as": "time_offset_min",
"calculate": "(datum.time - datum.base_time) / 60000"
},
{
"as": "eta_offset_min",
"calculate": "(datum.eta - datum.base_time) / 60000"
},
{
"as": "sta_offset_min",
"calculate": "(datum.sta - datum.base_time) / 60000"
},
{
"as": "delay_min",
"calculate": "datum.eta_offset_min - datum.sta_offset_min"
}
],
"mark": {
"type": "circle",
"filled": true
},
"width": 300,
"height": 225,
"encoding": {
"color": {
"value": "#202c40"
},
"x": {
"field": "time_offset_min",
"type": "quantitative",
"scale": {
"zero": false,
"type": "linear"
}
},
"y": {
"field": "eta_offset_min",
"type": "quantitative",
"scale": {
"zero": false,
"type": "linear"
}
}
}
}
Probably not useful, but I'm proud of what I'm building, so here is a screenshot of it.

[kanitw]

Thanks for reporting. Summarize transform actually generates a new data table (akin to an aggregate query in SQL).

What you want to do here is augmenting existing data table new a new summary column.
This could be done using the window transform in Vega, but we have not implemented it in Vega-Lite yet. (See #2488.)

As we're still revising the documentation for the 2.0 release, I'll note to make sure we make this clearer in the docs (#2279).

[1wiseash]

Thanks for your quick response. So, if I am reading it correctly, I can't do what I need directly in VL; I have to add the summary data to my dataset before using it to create the chart. Sorry if that is obvious. I just want to be sure before creating the pre-processing workaround.
Thanks again!
-Ashley

[kanitw]

Yes, you cannot do it directly in VL yet. Once #2488 is implemented, then you can. :)