vega-lite: Cannot create stacked bar chart without aggregate

I cannot seem to create a stacked chart with data that I have already pre-aggregated outside of vega-lite. I don’t know if this is by design or a bug. All examples of stacked bar/area charts I can find use some kind of aggregate in the y channel.

But the docs don’t mention this. From the docs on Stacked Bar Chart:

Adding color to the bar chart (by using the color attribute) creates a stacked bar chart by default."

Here is a simple spec with data that I would expect to create a stacked bar chart:

{
  "width": 300,
  "height": 300,
  "data": {
    "values": [
      {
        "dayOfWeek": 0,
        "payment_type": "Credit Card",
        "tips": 53
      },
      {
        "dayOfWeek": 0,
        "payment_type": "Cash",
        "tips": 93
      },
      {
        "dayOfWeek": 0,
        "payment_type": "No Charge",
        "tips": 4
      },
      {
        "dayOfWeek": 1,
        "payment_type": "Credit Card",
        "tips": 48
      },
      {
        "dayOfWeek": 1,
        "payment_type": "Cash",
        "tips": 71
      },
      {
        "dayOfWeek": 1,
        "payment_type": "No Charge",
        "tips": 1
      },
      {
        "dayOfWeek": 2,
        "payment_type": "Credit Card",
        "tips": 49
      },
      {
        "dayOfWeek": 2,
        "payment_type": "Cash",
        "tips": 77
      },
      {
        "dayOfWeek": 2,
        "payment_type": "No Charge",
        "tips": 1
      }
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {
      "field": "dayOfWeek",
      "type": "nominal"
    },
    "y": {
      "field": "tips",
      "type": "quantitative"
    },
    "color": {
      "field": "payment_type",
      "type": "nominal"
    }
  }
}

I’m getting this chart:

However if I add a aggregate: "sum" to the y channel (which is a no-op as all the combinations of payment_type and dayOfWeek are unique), it shows the expected result:

This seems to be an easy enough workaround, but I’d like to check if this is by design, in which case it would be great to document this behavior, e.g. “for stacked charts, you always need an aggregate”.