如何使用 mark_rule() 在 Altair 图表中显示一条垂直线

Question

I am creating an histogram using Python & Altair. I am able to include a vertical line as the mean, which works, but the code for the first Quartile (25th quantile) does not produce a vertical line.

I assume it is based that I use a numpy function to calculate the first quartile. But I am unsure how to do it differently.

What am I missing? Thank you!

import pandas as pd
import numpy as np
import altair as alt

df = pd.util.testing.makeDataFrame()

chart = (
    alt.Chart(df)
    .mark_bar()
    .encode(alt.X("A:Q", bin = True), y = "count()")
    .properties(width = 800, height = 300)
) 

# create mean rule ***WORKS***
mean = (
    alt.Chart(df)
    .mark_rule()
    .encode(
        x = "mean(A):Q"
    )
)

chart + mean

# create Q1 rule *** vertical line is NOT showing***
Q1 = (
    alt.Chart(df)
    .mark_rule()
    .encode(
        x = "np.quantile(A, 0.25):Q"
    )
)

chart + Q1

Any suggestions? Thank you!

Answer 1

Altair encoding strings do not parse arbitrary python code, so calling numpy functions will not work.

For quantiles in Altair, you can use thequantile transform . Here is an example with your data:

Q1 = (
    alt.Chart(df)
    .transform_quantile('A', probs=[0.25], as_=['prob', 'value'])
    .mark_rule()
    .encode(
        x = "value:Q"
    )
)

chart + Q1