# How to draw labeled parallel arrows in commutative diagram with TikZ?

I want to draw labeled parallel arrow in commutative diagram with TikZ, I searched on google and Tex Stackexchange, however, I found only the method to draw parallel arrow by transform canvas only, I tried to add the labeled but it failed.

Here is my picture:

I want to label a map from $\mathfrak{P}=\bigoplus_{i\inI}\mathfrak{P_{i}}$ to $\mathfrak{P_{i}}$ and from $\mathfrak{P_{i}}$ to $\mathfrak{P}=\bigoplus_{i\inI}\mathfrak{P_{i}}$, i.e the arrows of the square.

Here is the code :

\begin{tikzpicture}[node distance=2.8cm, auto]
\node (P) {$\mathfrak{P}=\bigoplus_{i\in I}\mathfrak{P_{i}}$};
\node(Q)[right of=P] {$\mathfrak{P_{j}}$};
\node (B) [below of=P] {$\mathfrak{B}$};
\node (C) [right of=B] {$\mathfrak{C}$};
\draw[transform canvas={yshift=0.5ex},->] (P) - -(Q);
\draw[transform canvas={yshift=-0.5ex},->](Q) -- (P);
\draw[->](Q) to node {$a$}(B);
\draw[->](Q) to node {$a$}(C);
\draw[->] (P) to node {$\pi_{j}$} (B);
\draw[->] (B) to node {$\psi$} (C);
\draw[->, dashed] (Q) to node [swap] {$g$} (B);
\end{tikzpicture}

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@percusse: Thank you for helping me editing my post. – Knumber10 Mar 16 '12 at 4:59

There are other ways to shift nodes but IMO the transform canvas is the most direct, so you are doing fine for that aspect.

To label a line, you can add a node command at the end of the line specification. The label may be placed above or below and you can specify where along the line with keywords such as midway or the more general pos=<fraction along line>. I also shifted the diagonal lines so that they may both be seen (since one was dashed).

I added a macro to simplify the shifting of the diagonal lines.

\documentclass[border=5pt]{standalone}

\usepackage{amsmath,amssymb}

\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}

\begin{tikzpicture}[node distance=2.8cm, auto]

\pgfmathsetmacro{\shift}{0.3ex}

\node (P) {$\mathfrak{P}=\bigoplus_{i\in I}\mathfrak{P_{i}}$};
\node(Q)[right of=P] {$\mathfrak{P_{j}}$};
\node (B) [below of=P] {$\mathfrak{B}$};
\node (C) [right of=B] {$\mathfrak{C}$};

\draw[transform canvas={yshift=0.5ex},->] (P) --(Q) node[above,midway] {\tiny top};
\draw[transform canvas={yshift=-0.5ex},->](Q) -- (P) node[below,midway] {\tiny bottom};
\draw[->](Q) to node {$a$}(C);
\draw[->] (P) to node[swap] {$\pi_{j}$} (B);
\draw[->,dashed] (B) to node {$\psi$} (C);

\draw[->,transform canvas={xshift=-\shift,yshift=\shift}](Q) to node {$a$}(B);
\draw[->, dashed,transform canvas={xshift=\shift,yshift=-\shift}] (Q) to node[swap] {$g$} (B);

\end{tikzpicture}

\end{document}


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([yshift=...]P)--([yshift=...]Q) is also an option. – percusse Mar 16 '12 at 4:06
@percusse: you are correct, but I think that that method is more appropriate when you are shifting each point by a different amount. – Frédéric Mar 16 '12 at 4:18
@Frédéric : Thank you very much! It works. – Knumber10 Mar 16 '12 at 4:57
With transform canvas one runs into two problems (not in this case, though): the bounding box is not correct, and any positions saved whilst under the influence will not be correct (try labelling the top to see). However, when I tried percusse's method, I got strange results. To get the right result, I had to add the anchor: ([yshift=2.5ex]P.east) -- ([yshift=2.5ex]Q.west). – Loop Space Mar 16 '12 at 7:43
I agree with Andrew for transform canvas. This option is sometimes dangerous (see the pgfmanual). Now ([yshift=...]P)--([yshift=...]Q) (remark from percusse) is wrong. You need to use P.east and Q.west. But for the arrow between Q and C, transform canvas is useful to get something elegant (similar arrows) – Alain Matthes Mar 16 '12 at 8:20

Perhaps to clarify some points.

1) First ([yshift=...]P)--([yshift=...]Q) does not work. Example:

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}
\node(A) at (0,0) {A};   \node(B) at (5,0) {B};
\draw[green] (A) -- (B);
\draw [yellow,yshift=.5 cm] (0,0) -- (5,0);
\draw[blue,yshift=1 cm] (A) -- (B); %problem
\draw[red] ([yshift=1 cm]A) -- ([yshift=1 cm]B);  %problem
\draw[magenta] ([yshift=1 cm]A.east) -- ([yshift=1 cm]B.west);
\end{tikzpicture}

\begin{tikzpicture}
\coordinate(A) at (0,0) ;   \coordinate(B) at (5,0);
\draw (A) -- (B);
\draw[red] ([yshift=.5 cm]A) -- ([yshift=.5 cm]B);  %fine
\draw[blue,yshift=1 cm] (A) -- (B); %problem
\end{tikzpicture}
\end{document}


2) About transform canvas

Canvas transformations should be used with great care. In most circumstances you do not want line widths to change in a picture as this creates visual inconsistency. Just as important, when you use canvas transformations pgf loses track of positions of nodes and of picture sizes since it does not take the effect of canvas transformations into account when it computes coordinates of nodes (do not, however, rely on this; it may change in the future). Finally, note that a canvas transformation always applies to a path as a whole, it is not possible (as for coordinate transformations) to use different transformations in different parts of a path. In short, you should not use canvas transformations unless you really know what you are doing.

I think it's better to avoid it

3) I remove right of etc. because i like to scale my pictures when it's necessary

right of seems to be more concise but you need to fix node distance=... and to write right of=P at (5,0) is more concise or --++(5,0)

\documentclass{scrartcl}
\usepackage{amsmath,amssymb}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}[auto,scale=2]

\node (P) at (0,0)  {$\mathfrak{P}=\bigoplus_{i\in I}\mathfrak{P_{i}}$};
\node (Q) at (3,0)  {$\mathfrak{P_{j}}$};
\node (B) at (0,-3) {$\mathfrak{B}$};
\node (C) at (3,-3) {$\mathfrak{C}$};

\draw[->] ([yshift =  .3ex]P.east)  --  node[above] {\tiny top}
([yshift =  .3ex]Q.west) ;
\draw[<-] ([yshift = -.3ex]P.east) --  node[below] {\tiny bottom}
([yshift = -.3ex]Q.west);
\draw[->](Q) to node {$a$}(C);
\draw[->] (P) to node[swap] {$\pi_{j}$} (B);
\draw[->,dashed] (B) to node {$\psi$} (C);

\draw[->]    ( Q.south west)  -- node[above] {$a$}
( B.north east) ;

\draw[->,dashed] ([xshift= 0.2ex ,yshift = - 0.3ex ] Q.south west) -- node[below] {$g$}
([xshift=  0.2ex ,yshift = - 0.3ex ]B.north east);

\end{tikzpicture}
\end{document}


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Wow, that's a strange output. I learned yet another subtle detail thanks to you. – percusse Mar 16 '12 at 16:21
There are a lot of strange outputs and subtle details with TeX and TikZ ! With my old neurons, I forget many of these details every day. – Alain Matthes Mar 16 '12 at 17:11

You might also want to have a look at the rather new package tikz-cd.

Its command \arrow has a mandatory argument identifying the direction, e.g. r for “right”. It also has an optional argument to specify TikZ options like transform canvas. It also has further argument pairs (optional and “mandatory” ) for labels.

• An arrow pointing right: \arrow{r}
• An arrow pointing right with label above: \arrow{r}{label}
• An arrow pointing right with label below: \arrow{r}[swap]{label}
• An arrow pointing right and shifted: \arrow[transform canvas={yshift=.5ex}]{r}

Here's a complete try:

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}[row sep=3cm,column sep=2.5cm]
\mathfrak{P}=\bigoplus_{i\in I}\mathfrak{P_{i}}
\arrow[transform canvas={yshift=.5ex}]{r}
\arrow[transform canvas={yshift=-.5ex},leftarrow]{r}
\arrow{d}[swap]{\pi_{j}}
& \mathfrak{P_{j}}
\arrow{d}{a}
\arrow[transform canvas={yshift=.3ex,xshift=-.3ex}]{ld}[swap]{g}
\arrow[transform canvas={yshift=-0.3ex,xshift=.3ex},dashed]{ld}{a} \\
\mathfrak{B}
\arrow{r}{\psi}
& \mathfrak{C}
\end{tikzcd}

\end{document}
`

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