Using indexing syntax here seems a bit surprising.  It's not actually accessing an element of the object (which is logically what `__getitem__` usually means).  While it's more verbose, I'd much prefer using a method, something like `ax.space('world')` or `ax.transform('world')`.  I think that's a lot more self descriptive.  I certainly, coming across this as part of the examples below, would be very surprised by this.  All the different ways in which it works (strings, wcs objects, transform objects) is great though.