Support alt.datum[signal_name] in addition to "datum[signal_name]"

[joelostblom]

Currently using alt.datum in altair does not support passing a signal/parameter whereas writing the expression string directly as JS-like string does (an example of dynamically passing fields via parameters here https://github.com/altair-viz/altair/pull/3357/files (both the chart title and calculate transform).

Here is another example:

import altair as alt
from vega_datasets import data

dropdown = alt.binding_select(
    options=['Horsepower', 'Displacement', 'Weight_in_lbs', 'Acceleration'],
    name='X-axis column '
)
xcol_param = alt.param(
    value='Horsepower',
    bind=dropdown
)

alt.Chart(data.cars.url).mark_circle().encode(
    x=alt.X('x:Q').title(''),
    y='Miles_per_Gallon:Q',
    color='Origin:N'
).transform_calculate(
    x=f'datum[{xcol_param}]'
    # x=alt.datum[xcol_param.name]
).add_params(
    xcol_param
)

The reason it fails is that alt.datum[xcol_param.name] inserts additional quotation around the parameter name in the VL spec:

"transform": [{"calculate": "datum['param_1']", "as": "x"}],

whereas the VL spec for f'datum[{xcol_param}]' does not:

"transform": [{"calculate": "datum[param_1]", "as": "x"}],

Open the Chart in the Vega Editor

Personally I think the cleanest would be if we could support just using the variable name without the .name suffix when using alt.datum, as in this example from the docs:

param_color_js_expr = alt.param(expr=f"{param_width.name} < 200 ? 'red' : 'black'")
param_color_py_expr = alt.param(expr=alt.expr.if_(param_width < 200, 'red', 'black'))

But even if that doesn't work, just supporting the use of the .name suffix without the quotes inserted would be great. Here are a few examples of how these datums and expressions get translated with differential quoting:

[jonmmease]

Thanks for writing this up, I think alt.datum[xcol_param] rather than alt.datum[xcol_param.name] makes good sense