Problem with arrow in xy

Question

I have some code with xy

\xymatrix@C=1pc@R=2.5pc{
  *+[F-]{a}
  \ar@(ul,ur)[]
  \ar@<-1.5ex>@{<-}[d]
  \ar@<1.5ex>@{<.}[d]
  \save+<0pc,1.1pc>*{\txt{$s$}} \restore
  \\
  *+[F-]{b}
  \ar@<0.2ex>@{<--}'[u] +D * {\bullet} []
            \save+<1.2pc,1.1pc>*{\txt{$t$}} \restore
}

that gives this image:

However, the two dashed arrows are two close together and make it difficult to read. I don’t know how to fix that. I tried adding @<0.2ex> to shift one of the dashed arrows. However it doesn’t solve the problem, and if I increase the value, the arrows are disconnected.

What I would like to have is an arrow starting at b, going up to a on the bullet, and going back to b. Is it possible?

score 3 · Accepted Answer · answered Sep 16 '22 at 10:01

Instead of using a bend you can use two single arrows, the first without an arrow tip (the 'going up' part) and the second with an arrow tip. Then you can position them individually, for example each at a distance of 0.3.

MWE:

\documentclass{article}
\usepackage[all]{xy}
\begin{document}
\xymatrix@C=1pc@R=2.5pc{
  *+[F-]{a}
  \ar@(ul,ur)[]
  \ar@<-1.5ex>@{<-}[d]
  \ar@<1.5ex>@{<.}[d]
  \ar@<0.3ex>@{--}[d]
  \save+<0pc,1.1pc>*{\txt{$s$}} \restore
  \\
  *+[F-]{b}
  \ar@<0.3ex>@{<--}[u] +D * {\bullet} []
            \save+<1.2pc,1.1pc>*{\txt{$t$}} \restore
}
\end{document}

Result:

Qrrbrbirlbel · Answer 2 · 2022-09-16T11:57:22.710

Here's the same diagram with the tikzcd package/library.

The key shift left and shift right can be used to shift the arrows orthogonally to their direction. I'll introduce the keys shl and shr that act as a factorized version of these.

The TikZ key /tikz/dot places the filled circle at the bottom of the cell. The TikZ-CD key /tikz/commutative diagrams/dot only sets the starting arrow tip (here a circle of diameter 2pt with a sep of -1pt so that it doesn't just touch the starting point).

I'm setting the default arrow tip of TikZ-CD to a thinner longer version since the original Computer Modern tip is rather wide, thanks to the bending library it can also bend to the path.

The first example uses the original loop above from TikZ which by default places the label above the line.

The second example uses la (loop above) which draws a wider loop but placed the label inside it.

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{
  cd,     % https://ctan.org/pkg/tikz-cd
  bending % for bended arrow tips
}
\tikzset{
  la/.style={swap,out=125,in=55,loop,looseness=5},
  dot/.style={append after command={
    node[circle,fill,inner sep=+0pt,minimum size=+1.5pt,name=\tikzlastnode-dot]
      at(\tikzlastnode.south){}}}}
\tikzcdset{
  shl/.style={shift left={(#1)*.56ex}}, shl/.default=1,
  shr/.style={shift right={(#1)*.56ex}}, shr/.default=1,
  dot/.code=\pgfsetarrowsstart{Circle[sep=-1pt,length=2pt]},
  tikzcd to/.tip={cm to[width=3pt, length=4pt, bend]},
  vh/.style={to path={|-(\tikztotarget)\tikztonodes}},
  hv/.style={to path={-|(\tikztotarget)\tikztonodes}},
  from dot leftdown/.style={from dot, start anchor=south west,
    end anchor={[xshift=-.5ex]}, yslant=1, hv},
  from dot rightdown/.style={from dot, start anchor=south east,
    end anchor={[xshift=.5ex]}, yslant=-1, hv},
}
\makeatletter
\tikzcdset{
  from dot/.style={from/.expanded=\tikzcd@ar@start-dot},
  to dot/.style={to/.expanded=\tikzcd@ar@target-dot}}
\makeatother
\begin{document}
\begin{tikzcd}[cells={nodes=draw}]
a \ar[loop above, "s"]
 \dar[dash, dashed, shl]
 \dar[dot,  dashed, shr] \\
b \uar[shl=2]
  \uar[shr=2, densely dotted, swap, "t" near start]
\end{tikzcd}
\begin{tikzcd}[cells={nodes=draw}]
|[dot]| a \ar[la, "s"]
         \dar[dashrightarrow, from dot leftdown]
         \dar[dash, dashed, from dot rightdown]
         \
b \uar[shl=2]
  \uar[shr=2, densely dotted, swap, "t" near start]
\end{tikzcd}
\end{document}

Problem with arrow in xy

2 Answers2

Code

Output