TODO

Random issues that need to be sorted out:

- Default limits

   Not all built-in layers provide their own limits, but some should
   (e.g., ypanel.loess(), but not ypanel.grid()).  Probably just the
   data limits, not computed, but something.

- Scales

   scales are currently ignored.

- Aspect ratio

   Need to fix aspect="xy" for one.

   More importantly, non-fill aspects place plot to the left of the
   available area, not middle, and leave a lot of space between plot
   and legends etc.

- switch.axes

   Fix axis labels (switch them too) when switch.axes = TRUE


.yagpenv -> .impenv

yagp.theme -> imp.theme