I'm working on a library that uses `altair`, and looking to make it an optional dependency. During this process, I thought `ChartType` and `is_chart_type` might be a good fit in `altair` itself.

I don't believe any existing `altair` code would directly benefit from these additions - but you could likely reduce some `isinstance` checks with [type narrowing](https://typing.readthedocs.io/en/latest/spec/narrowing.html) functions like `is_chart_type`.
For example `altair/vegalite/v5/api.py` has **63** occurrences of `isinstance`.

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per [CONTRIBUTING.md](https://github.com/vega/altair/blob/main/CONTRIBUTING.md)
> In particular, we welcome companion efforts from other visualization libraries to render the Vega-Lite specifications output by Altair. We see this portion of the effort as much bigger than Altair itself

Towards this aim, it could be beneficial to provide `typing` constructs/tools for downstream use.

Regarding my specfic use case, the below [Protocol](https://typing.readthedocs.io/en/latest/spec/protocol.html#runtime-checkable-decorator-and-narrowing-types-by-isinstance) - I believe - describes the concept of a `_/Chart`, for the purposes of writing to file:

```py
from typing import Any, Protocol, runtime_checkable

@runtime_checkable
class AltairChart(Protocol):
    data: Any

    def copy(self, *args: Any, **kwargs: Any) -> Any: ...
    def save(self, *args: Any, **kwargs: Any) -> None: ...
    def to_dict(self, *args: Any, **kwargs: Any) -> dict[Any, Any]: ...
    def to_html(self, *args: Any, **kwargs: Any) -> str: ...
```

Due to the number of symbols in `altair` and heavy use of mixins, coming to this conclusion can be a stumbling block.

However, exporting things like `ChartType`, `is_chart_type`, `AltairChart` could be a way to expose these shared behaviours - without requiring any changes to the core API.

`TopLevelMixin.save` calls a function accepting `pathlib` objects, but the two didn't share the same `Union` for `fp`.
When using `pyright`, the following warning is presented:

> Argument of type "Path" cannot be assigned to parameter "fp" of type "str | IO[Unknown]" in function "save"
> Type "Path" is incompatible with type "str | IO[Unknown]"
> "Path" is incompatible with "str"
> "Path" is incompatible with "IO[Unknown]"

This fix avoids the need to use type ignores on the public facing API, as a user.

By normalizing `fp` in `save`, it simplifies the signatures of two other functions and removes the need for `str` specific suffix handling.

…sion

Although there are branches it isn't needed, I think this is easier to read and maintain.

Very low priority change, just something I noticed while reading.

Believe these should be public, but it seems like the issue at build-time is that I'd be extending the public API.

…e duplication of list of charts

As [requested](https://github.com/vega/altair/pull/3420/files/a4de6d1feb6f3f17c7f6d4c7738eb008ff33d17e#r1606780541).
Seems to only occur in a few places, but all were autofix-able.

As [requested](vega@99b4f81#r1606785466)

An earlier commit assumed a minor optimization would be possible, by normalizing a `Union` early and reuse the narrowed type in "private" functions.

@binste [comment](https://github.com/vega/altair/pull/3420/files#r1610440552)