pola-rs/polars · error · ValueError

cannot use `plot.kde` when Series name is `'density'`

Error message

cannot use `plot.kde` when Series name is `'density'`

What it means

Series.plot.kde() encodes the y-axis with the Vega-Lite transform output field 'density' produced by transform_density. If the Series is named 'density', the source field and the generated density field collide, so polars raises ValueError upfront.

Solutions

1. Rename the Series: s.rename("value").plot.kde()
2. Use s.alias("measurement") before plotting
3. Pass a differently named Series at construction time

Example fix

// before
s.alias("density").plot.kde()
// after
s.alias("density_estimate").plot.kde()

if s.name == "density":
    s = s.rename("value")
s.plot.kde()

try:
    chart = s.plot.kde()
except ValueError:
    chart = s.rename("value").plot.kde()

When it happens

Trigger: Calling s.plot.kde() on a Series named exactly 'density' — often a Series that came from a previous density computation or was renamed to 'density'.

Common situations: Plotting KDE of a column produced by another KDE/density transform; data pipelines that normalize column names to statistical terms like 'density'.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Cannot call `plot. ` when Series name is 'index
• cannot call `plot.line` when Series name is 'index'
• cannot use `plot.hist` when Series name is `'count()'`
• altair>=5.4.0 is required for `.plot`
• Altair has no method 'mark_

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/9b5cd8517ac6d913. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/src/polars/series/plotting.py:114

        .. versionchanged:: 1.6.0
            In prior versions of Polars, HvPlot was the plotting backend. If you would
            like to restore the previous plotting functionality, all you need to do
            is add `import hvplot.polars` at the top of your script and replace
            `df.plot` with `df.hvplot`.

        Parameters
        ----------
        **kwargs
            Additional keyword arguments passed to Altair.

        Examples
        --------
        >>> s = pl.Series("price", [1, 3, 3, 3, 5, 2, 6, 5, 5, 5, 7])
        >>> s.plot.kde()  # doctest: +SKIP
        """  # noqa: W505
        if self._series_name == "density":
            msg = "cannot use `plot.kde` when Series name is `'density'`"
            raise ValueError(msg)
        encodings: Encodings = {"x": self._series_name, "y": "density:Q"}
        return (
            alt.Chart(self._df)
            .transform_density(self._series_name, as_=[self._series_name, "density"])
            .mark_area(tooltip=True)
            .encode(**encodings, **kwargs)
            .interactive()
        )

    def line(
        self,
        /,
        **kwargs: Unpack[EncodeKwds],
    ) -> alt.Chart:
        """
        Draw line plot.

        Polars does not implement plotting logic itself but instead defers to

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