Data transformers

Before a Vega-Lite or Vega specification can be passed to a renderer, it typically has to be transformed in a number of ways:

  • Pandas Dataframe has to be sanitized and serialized to JSON.
  • The rows of a Dataframe might need to be sampled or limited to a maximum number.
  • The Dataframe might be written to a .csv of .json file for performance reasons.

These data transformations are managed by the data transformation API of Altair.


The data transformation API of Altair should not be confused with the transform API of Vega and Vega-Lite.

A data transformer is a Python function that takes a Vega-Lite data dict or Pandas DataFrame and returns a transformed version of either of these types:

from typing import Union
Data = Union[dict, pd.DataFrame]

def data_transformer(data: Data) -> Data:
    # Transform and return the data
    return transformed_data

Built-in data transformers

Altair includes a default set of data transformers with the following signatures.

Raise a MaxRowsError if a Dataframe has more than max_rows rows:

limit_rows(data, max_rows=5000)

Randomly sample a DataFrame (without replacement) before visualizing:

sample(data, n=None, frac=None)

Convert a Dataframe to a separate .json file before visualization:

to_json(data, prefix='altair-data'):

Convert a Dataframe to a separate .csv file before visualiztion:

to_csv(data, prefix='altair-data'):

Convert a Dataframe to inline JSON values before visualization:



Multiple data transformers can be piped together using pipe:

from altair import pipe, limit_rows, to_values
pipe(data, limit_rows(10000), to_values)

Managing data transformers

Altair maintains a registry of data transformers, which includes a default data transformer that is automatically applied to all Dataframes before rendering.

To see the registered transformers:

>>> import altair as alt
>>> alt.data_transformers.names()
['default', 'json', 'csv']

The default data transformer is the following:

def default_data_transformer(data):
    return pipe(data, limit_rows, to_values)

The json and csv data transformers will save a Dataframe to a temporary .json or .csv file before rendering. There are a number of performance advantages to these two data transformers:

  • The full dataset will not be saved in the notebook document.
  • The performance of the Vega-Lite/Vega JavaScript appears to be better for standalone JSON/CSV files than for inline values.

There are disadvantages of the JSON/CSV data transformers:

  • The Dataframe will be exported to a temporary .json or .csv file that sits next to the notebook.
  • That notebook will not be able to re-render the visualization without that temporary file (or re-running the cell).

In our experience, the performance improvement is significant enough that we recommend using the json data transformer for any large datasets:


We hope that others will write additional data transformers - imagine a transformer which saves the dataset to a JSON file on S3, which could be registered and enabled as:

alt.data_transformers.register('s3', lambda data: pipe(data, to_s3('mybucket')))