design (timeseries charts): Improved loading state

[ericpgreen2]

TODO:

• [ ] Fix the timeseries split by dimensional values

Currently, our timeseries chart's "loading" state is a flat line at y=0.

This PR now grays-out the previous data to indicate a loading state.

Checklist:

• [ ] Covered by tests
• [ ] Ran it and it works as intended
• [ ] Reviewed the diff before requesting a review
• [ ] Checked for unhandled edge cases
• [ ] Linked the issues it closes
• [ ] Checked if the docs need to be updated
• [ ] Intend to cherry-pick into the release branch
• [ ] I'm proud of this work!