[UI] Hovering over chart should tell you value and time

__Proposal:__
Many small charts can be difficult to read individual points and timestamps. Hovering _anywhere_ on the chart should show you the data point in that vertical column, its value, the timestamp it occurred at, and visualize it with a vertical bar.

__Use case:__
Difficult to read times on charts that are not full-page width, and difficult to get detail on exact values.

No files, tests, or entry points are named. Start by locating the small-chart UI component and its existing hover behavior, then confirm the implementation path with maintainers. Done means hovering anywhere on a chart shows the aligned data point, its value and timestamp, and a vertical bar.

Written by the indexing model from the issue text.