Highlight toggle flips states when graph is updated

When hovering over the description of a graph, its line and area get highlighted by adding a specific CSS classes to these elements in DOM. When the pointer leaves the description, these classes are removed from the DOM.

If a graph is highlighted while being updated with the latest data, the toggle flips. Now the line and area of a graph are highlighted when the pointer leaves the description.

Such thing is unlikely to happen when a graph refreshes in long periods of time, but can be a problem if the graph updates itself every five seconds or so.

This pull request takes this edge case into account. I also did some formatting.