(a current Altair maintainer and a former plotly.py maintainer here)

Plotly is definitely a great option as well, and it can do a bunch of things Vega-Altair is not designed for. One comment, just in case you weren't aware, is that there is a relatively new library that provides good integration between Altair and Dash: https://github.com/altair-viz/dash-vega-components. It even makes it possible to access Altair selection states in Dash callbacks so that you can have other dashboard components respond to selections.