Code Insights: improve testing for Line Charts

Created by: unclejustin

We are having multiple bugs creeping up with line charts. The complexity has grown such that manual testing can no longer cover every scenario. We need to identify all of the possible states and add unit testing for these issues.

Related issues that should have been caught:

• https://github.com/sourcegraph/sourcegraph/issues/38304
• https://github.com/sourcegraph/sourcegraph/issues/38305

This PR is also a good example of bugs being introduced mid-PR update. Many revisions and round trips to design for testing because there are just too many scenarios to account for manually with every change.