Tikz matrix vertical alignment

Question

I'm having trouble with the vertical alignment of the nodes in a tikz matrix. I have two plots next to each other. I want them to be aligned with respect to the center of their bounding boxes, but they end up aligned with respect to the zero coordinate (the abscissa in this case). Based on other questions (e.g., Aligning matrix nodes in tikz), I've tried with various combinations of align=base, align=center, anchor=center, etc, without success.

Below is the code and corresponding output.

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{figure}[h!]
\centering
\begin{tikzpicture}
\matrix [column sep=1cm, row sep=1cm, cells={xscale=0.7, yscale=2},
align=center, nodes={rectangle, anchor=center, align=center}]
{
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,0) -- (0,1.3);
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {1/sqrt(1 + \x*\x)});
  &
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,-0.6) -- (0,0.6);
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {-0.4*pi*atan(\x)/180});
  \\
};
\end{tikzpicture}
\caption{How to align?}
\end{figure}
\end{document}

Any idea about how to vertically align the center of the cell's bounding boxes?

Answer 1

You don't need a matrix, just declare each graphic as a separate tikzpicture which will be treated as any other box in LaTeX. No need for \makebox.

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{figure}[h!]
\centering
\begin{tikzpicture}[xscale=0.7,yscale=2]
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,0) -- (0,1.3);   
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {1/sqrt(1 + \x*\x)});
\end{tikzpicture}
\hfill
\begin{tikzpicture}[xscale=0.7,yscale=2]
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,-0.6) -- (0,0.6);
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {-0.4*pi*atan(\x)/180});
\end{tikzpicture}
\caption{How to align?}
\end{figure}
\end{document}

Answer 2

You can achieve the alignment by using the tikzlibrary matrix, adding matrix of nodes to your matrix options, and drawing your plots as nested tikzpictures.

Here the full code

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}

\begin{document}
\begin{figure}[h!]
    \centering
    \begin{tikzpicture}
    \matrix [matrix of nodes,column sep=1cm, row sep=1cm, align=center,
        nodes={rectangle, anchor=center, align=center}]
    {
    \begin{tikzpicture}[xscale=0.7, yscale=2]
        \draw [->] (-4.2,0) -- (4.2,0);
        \draw [->] (0,0) -- (0,1.3);
        \draw [blue, ultra thick, domain=-4:4, samples=100] plot
        (\x, {1/sqrt(1 + \x*\x)});
    \end{tikzpicture}
    &
    \begin{tikzpicture}[xscale=0.7, yscale=2]
        \draw [->] (-4.2,0) -- (4.2,0);
        \draw [->] (0,-0.6) -- (0,0.6);
        \draw [blue, ultra thick, domain=-4:4, samples=100] plot
        (\x, {-0.4*pi*atan(\x)/180});
    \end{tikzpicture}
    \\
    };
    \end{tikzpicture}
    \caption{How to align?}
\end{figure}
\end{document}

And the result

Answer 3

You can put the first graph inside a scope and lower it by -.65cm (i.e., to the mid point of y-axis).

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{figure}[h!]
\centering
\begin{tikzpicture}
\matrix [column sep=1cm, row sep=1cm, cells={xscale=0.7, yscale=2},
align=center, nodes={rectangle, anchor=center, align=center}]
{
   \begin{scope}[yshift=-0.65cm]
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,0) -- (0,1.3);   
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {1/sqrt(1 + \x*\x)});
  \end{scope}
  &
  \draw [->] (-4.2,0) -- (4.2,0);
  \draw [->] (0,-0.6) -- (0,0.6);
  \draw [blue, ultra thick, domain=-4:4, samples=100] plot
  (\x, {-0.4*pi*atan(\x)/180});
  \\
};
\end{tikzpicture}
\caption{How to align?}
\end{figure}
\end{document}