How to change attribute based on boolean condition

Question

I am trying to alter point size based on whether its name exists in a list or not, I've tried many different ways but I keep generating this error.

Code:

    graph = alt.Chart(df).mark_point( filled = False).encode(
        x=alt.X(axe_x),
        y=alt.Y(axe_y),
        size=alt.condition(
            (alt.datum.name) in (some_list),
            alt.value(150),
            alt.value(50))

    )

Error: NotImplementedError: condition predicate of type <class 'bool'>

How can I get around this?

Answer 1

You can use a transform_lookup for this

import altair as alt
import pandas as pd
from vega_datasets import data
source = data.cars()
# lookup table matching the string to corresonding size
df2 = pd.DataFrame({
    'key': ['Europe', 'Japan', 'USA'],
    's': [50, 50, 200]
})

alt.Chart(source).mark_circle(opacity=0.5).transform_lookup(
  lookup='Origin',
  from_=alt.LookupData(df2, key='key', fields=['s'])    
).encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
    size =alt.Size('s:N', title='Size')
)