- Show labels at a readable size, whatever the zoom.
- Keep labels within the screen bounds where possible rather than letting them scroll off.
- Fade out large-scale features (e.g. arrows spanning long time frames) when you get close, so they don't blot out everything in between.
- Project y values through tanh so you can see everything at once, but focus on the bit you're interested in.
Of course, there are other opportunities for adding interaction too.