How can I move the arrow inside the commutative diagram into the centre?

Question

I have the following diagram:

\documentclass{article}
\usepackage[all]{xy}
\begin{document}
\xymatrix@R.8pc@C.8pc{
F(Y) \ar[rr]^{E_{Y}} \ar[dd]_{F(f)} && F(Y) \ar[dd]^{F(f)} \\
 & \mbox{\phantom{$x$}} \ar@{<=}[ur]^{\alpha_{f}}  && \\
F(X) \ar[rr]_{E_{X}} && F(X)
}
\end{document}

I wish to get the arrow inside the diagram to be placed in the centre. Is there a way to do this in the xy package without using dvips or 2cell?

Answer 1

This might be what you're after - I've added an additional row and column to the \xymatrix, which allowed to move the arrow to the centre:

\documentclass{article}
\usepackage[all]{xy}% http://ctan.org/pkg/xypic
\begin{document}
\xymatrix@R.8pc@C.8pc{
  F(Y) \ar[rrr]^{E_{Y}} \ar[ddd]_{F(f)} &&& F(Y) \ar[ddd]^{F(f)} \\
  &&& \\
  & \ar@{<=}[ur]^{\alpha_{f}} && \\
  F(X) \ar[rrr]_{E_{X}} &&& F(X)
}
\end{document}