View data after transformation

user_13724418 0
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Is it possible to see the data after Altair applies transformations and aggregations?

For example, can you access the underlying data after the following transformations?

import altair as alt
from vega_datasets import data

source = data.seattle_weather.url

step = 20
overlap = 1

alt.Chart(source, height=step).transform_timeunit(
    Month='month(date)'
).transform_joinaggregate(
    mean_temp='mean(temp_max)', groupby=['Month']
).transform_bin(
    ['bin_max', 'bin_min'], 'temp_max'
).transform_aggregate(
    value='count()', groupby=['Month', 'mean_temp', 'bin_min', 'bin_max']
).transform_impute(
    impute='value', groupby=['Month', 'mean_temp'], key='bin_min', value=0
).mark_area(...
)

Above code from the Ridgeplot example

altair python
1 Answers

Transforms are evaluated in Javascript, and there is not any built-in way to access data in the javascript frontend from the Python backend. However, there is an experimental package called altair_transform that is able to evaluate most Vega expressions in Python.

For your chart you can use it like this:

import altair as alt
from vega_datasets import data

source = data.seattle_weather()

step = 20
overlap = 1

chart = alt.Chart(source, height=step).transform_timeunit(
    Month='month(date)'
).transform_joinaggregate(
    mean_temp='mean(temp_max)', groupby=['Month']
).transform_bin(
    ['bin_max', 'bin_min'], 'temp_max'
).transform_aggregate(
    value='count()', groupby=['Month', 'mean_temp', 'bin_min', 'bin_max']
).transform_impute(
    impute='value', groupby=['Month', 'mean_temp'], key='bin_min', value=0
).mark_area().encode(
    x='Month:T',
    y='value:Q'
)

import altair_transform
data = altair_transform.extract_data(chart)
print(data)

     bin_min      Month  mean_temp  bin_max  value
0        0.0 1900-01-01   8.229032     -5.0    2.0
1        5.0 1900-01-01   8.229032      0.0   19.0
2       10.0 1900-01-01   8.229032      5.0   72.0
3       15.0 1900-01-01   8.229032     10.0   29.0
4       20.0 1900-01-01   8.229032     15.0    2.0
..       ...        ...        ...      ...    ...
103     20.0 1900-12-01   8.194355     15.0    4.0
104     25.0 1900-12-01   8.194355      NaN    0.0
105     30.0 1900-12-01   8.194355      NaN    0.0
106     35.0 1900-12-01   8.194355      NaN    0.0
107     40.0 1900-12-01   8.194355      NaN    0.0

[108 rows x 5 columns]
user_29378310
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