Add altair to notebook dependencies

Author: rlskoeser

pyproject.toml

@@ -49,7 +49,7 @@ classifiers = [
 [project.optional-dependencies]
 docs = ["sphinx>=7.0.0", "alabaster", "myst-parser", "myst-parser[linkify]"]
 test = ["pytest>=7.2", "pytest-ordering", "pytest-cov"]
-notebooks = ["jupyterlab", "pandas", "treon"]
+notebooks = ["jupyterlab", "pandas", "treon", "altair"]
 check = ["undate[docs]", "undate[notebooks]", "mypy", "ruff"]
 dev = [
     "pre-commit>=2.20.0",