Animate lines with y-scale transitions

This changes up the animation strategy a bit:

1. Add a new point to the end of each series
2. Update the X domain, dropping the oldest point
3. Draw all the lines using the new X domain, but translated to stay in
   their old positions
4. Update the Y domain, dropping the oldest point
5. Update the X & Y axes with the new X & Y scales
6. Animate (transition) the lines & axes simultaneously, redrawing the
   line with the new Y scale allowing it to grow/shrink during the
   transition, and dropping the translation from (3).