Enables a tooltip on the graph to make it easier to see what point in time a part of the graph corresponds to.
This PR includes the change in #484, but also deals with the potential issues that the simplicity of 484 introduced.
TODO:
- make it easier to select a point on the graph by factoring in the vertical position of the mouse pointer also.
- maybe - add ridges on the X-axis to show where an interval change or time jump occurs.
- make the Units translateable.