Independent encode of radius and theta

[qnzhou]

Hi there,

I am playing with this example using arc as mark. While the code works well as it is, however, if I assign radius to another column in the data frame, the plot looks completely wrong. Here is an example:

source = pd.DataFrame({"values": [12, 23, 47, 6, 52, 19]})
source["radius"] = source.values # <-- Create a new column for radius.

base = alt.Chart(source).encode(
    alt.Theta("values:Q").stack(True),
    alt.Radius("radius:Q").scale(type="sqrt", zero=True, rangeMin=20),
    color="values:N",
)

base.mark_arc(innerRadius=20, stroke="#fff")

Here is what I get:

I am using the latest atlair version 5.1.1. Any clues on why it is the case?

[joelostblom]

Hmm, not sure what happens there, but it is related to how Vega-Lite renders the chart. It works if you set both columns to 'radius' too. You can report it in the Vega-Lite repo by clicking the three dots in the top right corner and creating a link to the sped in the Vega Editor

[qnzhou]

With opacity turned to 0.5, it reveals more info about the chart. I think the stack=True setting for theta channel is not respected.

Digging a bit deeper into the compiled vega-lite json setting, I see the following stack transform:

        {
          "type": "stack",
          "groupby": ["radius"],
          "field": "values",
          "sort": {"field": ["values"], "order": ["ascending"]},
          "as": ["values_start", "values_end"],
          "offset": "zero"
        },

Note that it contains a groupby setting that is set to ["radius"] (instead of [] when both radius and theta uses the same data), which seems incorrect. This also shows the problem is probably with Altair instead of Vega-Lite.

[joelostblom]

Nice find digging deeper! Since the issue occurs in the Compiled Vega spec, it means something is likely wrong in the translation of the Vega-Lite spec to the Vega spec. This happens on the Vega-Lite side of things, Altair only renders Vega-Lite specification, but does not control how this spec is translated into a Vega spec

[qnzhou]

Thanks! It seems this bug has been reported back Feb 2022 but has not been fixed in Vega-Lite. This bug unfortunately makes altair/vega-lite incapable of creating radial chart.

[joelostblom]

Thanks for digging up that issue. I will let the discussion continue there and close this one since there is nothing we can do on the Altair side of things to fix this.

Edit: No need to close this I guess since it is a discussion and not an issue, but I marked your answer as the "solution"