Use event.which for identifying the key triggered the event.
Fix misplaced style elements.
Display column numbers over magnified panels.
Use a sans-serif font by default.
Delete unnecessary arguments.
Set the delay of the initial magnification animation to 0.
Add shadow to a magnified row number.
Use visibility property instead of colors.
Remove transition-related properties of '.panel:before'.
Use 'display: none' to hide column numbers.

I felt that its animation got a little bit smoother, but I'm not sure.
Refactoring.
Initially put the data panel at (0, 0).
Make pull/push bars look 3D.
Show/hide the data panel by ESC key.