I'm using Plotly more lately, for 2 reasons - chart have very good interactivity by default and I'm also a Dash user - so it comes in naturally.
But I believe Altair is excellent for exploration - simple charts are very simple to create, and it's not complicated to create a complex exploration dashboard - with all charts linked between them - so you can select a subset of data on one chart, and it automatically updates the others. Highly recommend at least exploring it !
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.
Wasn't aware of that. Are there any plans to improve the interactivity - by that I mean scale on X / Y independently - 'plotly' like ? I know you can bind specifically to one axis, when generating the chart, however, what I usually want is to be able to just zoom in on a specific axis when looking at the chart - Plotly is really great as the behaviour is depending of what you select - a rectangle is just a rectangle (zoom on both axes), a vertical selection zooms on Y axis, and horizontal one on X axis.
I'm also using it in a regular React app and it has quite decent APIs.