Graph track plot highlighting

## Description

Highlight currently selected plot on hover, to help in scenarios where the plot density is large

![Image](https://github.com/user-attachments/assets/470be547-325b-4b91-874e-54cfe0cef261)

## How to test

Load a graph track with a large number of plots or dips

## Acceptance criteria

1. A plot/dip should be highlighted when the user hovers over it
