feat: add warning for deduplications

[mattijn]

Attempt to fix #3891.

This PR now emits a warning when identical parameters are merged during concatenation to alert users to unintended linking.

With #3851, we now use a hash-based naming for parameters and views for deterministic names and thread safety, but I said in this comment #3851 (comment) that duplicate explicit names will raise an error to catch potential mistakes.

• Identical parameters gets deduplicated only when a hash-based name was derived
• Explicit parameter names are still supported, but duplicate explicit names will raise an error to catch potential mistakes.

But I found that this only works for a single chart (via add_params): hash-based duplicates are dropped; explicit duplicates raise. During concatenation (via _combine_subchart_params) all identical parameters (hash-based and explicit) are deduplicated and a warning is raised, so when using explicit names there is no raising of error.

Eg. this (explicit name for interactive selection paramater, that becomes identical during concatenation):

import altair as alt
from altair.datasets import data

cars = data.cars.url

chart = (
    alt.Chart(cars)
    .mark_point()
    .encode(x="Horsepower:Q", y="Miles_per_Gallon:Q")
    .interactive(name="MY_CHART")
)
(chart & chart).to_dict()

Returns

/var/folders/b3/pctp5dwn6t7fm1t4sc4tknb00000gp/T/ipykernel_88674/3413242154.py:12: 
UserWarning: Automatically deduplicated selection parameter with identical configuration. 
If you want independent parameters, explicitly name them differently (e.g., name='param1', name='param2'). 
See https://github.com/vega/altair/issues/3891
  (chart & chart).to_dict()

{'config': {'view': {'continuousWidth': 300, 'continuousHeight': 300}},
 'vconcat': [{'mark': {'type': 'point'},
   'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'},
    'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}},
   'name': 'view_6e7cfb454e831ee6_0'},
  {'mark': {'type': 'point'},
   'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'},
    'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}},
   'name': 'view_6e7cfb454e831ee6_1'}],
 'data': {'url': 'https://cdn.jsdelivr.net/npm/[email protected]/data/cars.json'},
 'params': [{'name': 'MY_CHART',
   'select': {'type': 'interval', 'encodings': ['x', 'y']},
   'bind': 'scales',
   'views': ['view_6e7cfb454e831ee6_0', 'view_6e7cfb454e831ee6_1']}],
 '$schema': 'https://vega.github.io/schema/vega-lite/v6.1.0.json'}

And this (explicit identical name for parameters):

from altair.datasets import data

cars = data.cars.url
param_opacity = alt.param(value=1, name='MY_PARAM')
param_size = alt.param(value=1, name='MY_PARAM')

# Create chart with same parameter added twice
chart = (
    alt.Chart(cars)
    .mark_circle(opacity=param_opacity, size=param_size)
    .encode(x="Horsepower:Q", y="Miles_per_Gallon:Q", color="Origin:N")
    .add_params(param_opacity, param_size)
)

Returns

ValueError: Duplicate explicit parameter name: MY_PARAM

I'm merely documenting this. I don't know what to do with it, since I'm not really sure if we can improve this situation any better.

It would be great if you could have a look to @joelostblom!