# How to align TikZ figures in subfigs

I use subfig to show an array of tikz figures. Unfortunately these figures are sometimes of different height, so they don't show up properly aligned:

I'd like to see all nodes on the same line. I'm new to TikZ, so I'm wondering if it is possible to give all figures a minimum height. I tried to set a bounding box using \useasboundingbox (0,0) rectangle (5,2); but it was difficult for me to estimate the correct values. Is there a way to calculate that, or is it possible to render an invisible phantom edge (with label) that gives the figure a) the same height as figure b)? Or is there a way to do the alignment on subfig's side?

Below is a MWE:

\documentclass{article}
\usepackage{subfig}
\usepackage{amsmath}
\usepackage{tikz}

\usetikzlibrary{arrows}
\tikzstyle{state}=[circle,thick,draw=black!75,fill=black!20,minimum size=6mm, text=black]

\begin{document}

\begin{figure}
\centering
\subfloat[In-only]{\label{fig:in}
\begin{tikzpicture}[node distance=2.8cm, bend angle=35,auto]
\node [state] (v1) at (1,0) {$D$};
\node [state] (v2) at (4,0) {$N$};
\draw (v1) edge [<-, bend left] node {$M_\text{in}$} (v2);
\end{tikzpicture}
}
\subfloat[In-only]{\label{fig:in}
\begin{tikzpicture}[node distance=2.8cm, bend angle=35,auto]
\node [state] (v1) at (1,0) {$D$};
\node [state] (v2) at (4,0) {$N$};
\draw (v1) edge [<-, bend left] node {$M_\text{in}$} (v2);
\draw (v1) edge [->, bend right] node {$M_\text{out}$} (v2);
\end{tikzpicture}
}

\end{figure}
\end{document}

-
A possible hack-ish solution which also requires a little bit of adjustment is adding baseline=1cm to the first tikzpicture options. I don't know too much about subfig but you can also consider subcaption package or using minipages :) See further vertical alignment with subfig and subcaption vs. subfig –  percusse Dec 15 '11 at 13:35

For each TikZ picture, set the baseline option and refer to the node you would like to use for the alignment:
\begin{tikzpicture}[baseline=(nodename.base),...]

Here baseline=(v1.base) because v1 and v2 are aligned vertically in each picture. –  Alain Matthes Dec 15 '11 at 14:11