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.. currentmodule:: altair
.. _user-guide-density-transform:
Density
~~~~~~~
The density transform performs one-dimensional
`kernel density estimation <https://en.wikipedia.org/wiki/Kernel_density_estimation>`_
over input data and generates a new column of samples of the estimated densities.
Here is a simple example, showing the distribution of IMDB ratings from the movies
dataset:
.. altair-plot::
import altair as alt
from vega_datasets import data
alt.Chart(data.movies.url).transform_density(
'IMDB_Rating',
as_=['IMDB_Rating', 'density'],
).mark_area().encode(
x="IMDB_Rating:Q",
y='density:Q',
)
The density can also be computed on a per-group basis, by specifying the ``groupby``
argument. Here we split the above density computation across movie genres:
.. altair-plot::
import altair as alt
from vega_datasets import data
alt.Chart(
data.movies.url,
width=120,
height=80
).transform_filter(
'isValid(datum.Major_Genre)'
).transform_density(
'IMDB_Rating',
groupby=['Major_Genre'],
as_=['IMDB_Rating', 'density'],
extent=[1, 10],
).mark_area().encode(
x="IMDB_Rating:Q",
y='density:Q',
).facet(
'Major_Genre:N',
columns=4
)
Transform Options
^^^^^^^^^^^^^^^^^
The :meth:`~Chart.transform_density` method is built on the
:class:`~DensityTransform` class, which has the following options:
.. altair-object-table:: altair.DensityTransform
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