Align independent tikzpictures

Question

I have two tikzpictures:

\documentclass{article}
\usepackage{tikz}
\usepackage{subcaption}
\usetikzlibrary{shapes,arrows}
\begin{document}
\pagestyle{empty}

\tikzstyle{notarget} = [rectangle, rounded corners, minimum
  width=1cm, minimum height=1cm,text centered, draw=none,
  opacity=0,text opacity=1,align=center]
\tikzstyle{target} = [rectangle, rounded corners, minimum width=1cm, minimum
  height=1cm,text centered, draw=black, very thick,text
  opacity=1,align=center]

\begin{figure}[ht]
  \centering
  \begin{subfigure}[b]{0.3\textwidth}
    \begin{tikzpicture}[node distance=1.2cm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, right of=n3] {girl\\NN};
%      \draw [thick,->] (n4) -- ++(1cm,0);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
  \qquad\qquad
  \centering
  \begin{subfigure}[b]{0.3\textwidth}
    \begin{tikzpicture}[node distance=1.2cm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, below of=n3,anchor=north] {girl\\NN};
      \draw [thick,->] (n4) -- (n3);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
  \label{fig:rightaction}
\end{figure}
\end{document}

But I do not know how to align the to figures vertically, currently it shows like this:

I would like to align it like this:

I have tried the following answers, without success:

• cannot figure out how to produce some tikz pictures next to each other
• Inline TikZ - vertical centering
• How to make a tikz node, top-aligned with the rest of the text?

UPDATE

I get it working with the help of Guilherme, here is:

\begin{figure}[ht]
  \centering
  \begin{subfigure}[b]{0.4\textwidth}
    \begin{tikzpicture}[node distance=2mm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right=of n1] {saw\\VBD};
      \node (n3) [target, right=of n2] {a\\DT};
      \node (n4) [target, right=of n3] {girl\\NN};
      \node (n4') [target, right=of n4,draw=none,anchor=west] {};
      \node (n5') [target, below=5mm of n4',anchor=north,draw=none] {};
      \draw [thick,->] (n5'.center) -- ++(5mm,0);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
  \qquad\qquad
  \centering
  \begin{subfigure}[b]{0.4\textwidth}
    \begin{tikzpicture}[node distance=2mm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right=of n1] {saw\\VBD};
      \node (nn) [notarget,draw=none,right=of n2] {};
      \node (n3) [target, right=of nn] {girl\\NN};
      \node (n4) [target, below=5mm of n3] {a\\DT};
      \draw [thick,->] (n4) -- (n3);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
\end{figure}

And the result:

UPDATE 2

Finally I've used gemot's solution:

\begin{figure}[ht]
  \centering
  \begin{tabular}{p{0.4\textwidth}cp{0.4\textwidth}}
    \begin{tikzpicture}[node distance=2mm,baseline=(n3)]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right=of n1] {saw\\VBD};
      \node (n3) [target, right=of n2] {a\\DT};
      \node (n4) [target, right=of n3] {girl\\NN};
    \end{tikzpicture}
  &
    \tikz\draw [thick,->] (0,0) -- ++(.5cm,0);
  &
    \begin{tikzpicture}[node distance=2mm,baseline=(n3)]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right=of n1] {saw\\VBD};
      \node (n3) [target, right=of n2] {a\\DT};
      \node (n4) [target, below=5mm of n3,anchor=north] {girl\\NN};
      \draw [thick,->] (n4) -- (n3);
    \end{tikzpicture}
  \\
    \subcaption{}
  &&
    \subcaption{}
  \end{tabular}
  \label{fig:rightaction}
\end{figure}

Answer 1

Here is a solution that does not require phantom drawings.

• Use the baseline option of tikzpicture to specify the horizonal alignment.
• Use some tabular environment for the overall arrangement. Because of the captions, you have to use p columns or some other type creating a box.

\documentclass{article}
\usepackage{tikz}
\usepackage{subcaption}
\usetikzlibrary{shapes,arrows}
\begin{document}
\pagestyle{empty}
\tikzset
  {common/.style =
    {rectangle, rounded corners, minimum width=1cm, minimum height=1cm,
     text centered, text opacity=1, align=center
    }
  ,notarget/.style = {common, draw=none, opacity=0}
  ,target/.style   = {common, draw=black, very thick}
  }

\begin{figure}[ht]
  \centering
  \begin{tabular}{p{0.4\textwidth}cp{0.4\textwidth}}
    \begin{tikzpicture}[node distance=1.2cm,baseline=(n3)]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, right of=n3] {girl\\NN};
    \end{tikzpicture}
  &
    \tikz\draw [thick,->] (0,0) -- ++(1cm,0);
  &
    \begin{tikzpicture}[node distance=1.2cm,baseline=(n3)]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, below of=n3,anchor=north] {girl\\NN};
      \draw [thick,->] (n4) -- (n3);
    \end{tikzpicture}
  \\
    \subcaption{before}
  &&
    \subcaption{after}
  \end{tabular}
  \caption{A transformation}\label{fig:rightaction}
\end{figure}
\end{document}

Answer 2

There are probably lots of ways to do this. Two I can see easily implemented: (i) use a \savebox and \raisebox or (ii) do the same drawing, but invisible (with \path). I'll show only the second because I think it's considerably less complicated and, as far as I know, the \savebox/\raisebox solution is already in the site.

\documentclass{article}
\usepackage{tikz,calc}
\usepackage{subcaption}
\usetikzlibrary{shapes,arrows}
\begin{document}
\pagestyle{empty}

\tikzset{notarget/.style={rectangle, rounded corners, minimum
  width=1cm, minimum height=1cm,text centered, draw=none,
  opacity=0,text opacity=1,align=center},
    target/.style={rectangle, rounded corners, minimum width=1cm, minimum
  height=1cm,text centered, draw=black, very thick,text
  opacity=1,align=center}}

\begin{figure}[ht]
  \centering
  \begin{subfigure}[b]{0.3\textwidth}
    \begin{tikzpicture}[node distance=1.2cm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, right of=n3] {girl\\NN};
      \node (n4') [target, below of=n3,anchor=north,draw=none] {};% <<<<<<< INVISIBLE
      \path [thick,->] (n4) -- (n3);% <<<<<<<<<<< INVISIBLE
%      \draw [thick,->] (n4) -- ++(1cm,0);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
  \qquad\qquad
  \centering
  \begin{subfigure}[b]{0.3\textwidth}
    \begin{tikzpicture}[node distance=1.2cm]
      \node (n1) [notarget] {I\\PRP};
      \node (n2) [notarget, right of=n1] {saw\\VBD};
      \node (n3) [target, right of=n2] {a\\DT};
      \node (n4) [target, below of=n3,anchor=north] {girl\\NN};
      \draw [thick,->] (n4) -- (n3);
    \end{tikzpicture}
    \caption{}
  \end{subfigure}
  \label{fig:rightaction}
\end{figure}
\end{document}