This commit:
1. Makes the vertical tooltip line follow the frequency bin rather than taking up the entire vertical.
2. Makes the tooltip aligned with the pivot rather than the mouse point. This prevents the tooltip from occluding the marker line.
3. Makes the tooltip styling match the dark background / white text used in tree tooltips. This increases aesthetic consistency.

This commit updates two aspects of projection frequencies:
1. Adds a "Projection" label centered over the projected portion of the frequencies streamgraph
2. Changes tooltip labeling to "Projected frequency" when appropriate