altair - 如何根据下拉列表为图表选择数据？

Question

我有一个数据框，我想根据下拉菜单对其进行子选择。所以给定以下代码:

import pandas as pd
import altair as alt
from vega_datasets import data

cars = pd.melt(data.cars(), ['Horsepower', 'Origin', 'Name', 'Year'])
cars.head()

select_box = alt.binding_select(options=list(cars['variable'].unique()))
selection = alt.selection_single(name='d_axis', fields=['variable'], bind=select_box)

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='value',
    color='Origin',
    tooltip='Name'
).add_selection(
    selection
).transform_filter(
    selection
)

我不想更改 X 值，而只想选择 Origin 为“USA”的汽车。
所以实际上在哪里

alt.Chart(cars).mark_point().encode(

变成:

alt.Chart(cars[cars['Origin'] == selection]).mark_point().encode(

谢谢，
斯蒂芬

Answer 1

您可以将选择绑定(bind)到要过滤的字段:

import altair as alt
from vega_datasets import data

cars = data.cars()

select_box = alt.binding_select(options=list(cars['Origin'].unique()))
selection = alt.selection_single(fields=['Origin'], bind=select_box)

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon'
).add_selection(
    selection
).transform_filter(
    selection
)