I don't understand the user interaction you are looking for but to implement custom clients, you can look at these docs: https://idl.uw.edu/mosaic/web-clients/.

My understanding is that you'd like users to be able to:

1. Draw out an interval between two points (a, b) along an axis
2. Display the distance between the points in data units ((b - a) or a date diff)

I don't think this is achievable with the current declarative specs. You could create a param and bind an interval selection to it to capture a user selected interval, but there's no way to then calculate the distance between interval end points within the spec.

However, if you are using the JS API (not a YAML or JSON spec) there should be some relatively simple things you could do:

• Listen to changes to the interval "span" param and update two other params (one for each endpoint) that then drive updates to other Mosaic plots, or
• Add your own web component that simply listens to changes on the interval span param and updates the displayed distance. I don't think any other integration with the Mosaic architecture would be needed.