Making curved path appear parallel in relation to a TikZ node

I have a diagram which looks like this:

I have horizontally aligned nodes b and c with regards to their centers, but this causes an issue with the curved path z due to the fact it is no longer symmetrical.

How can I make the ends of the path z to be parallel to one another and the label align with node a, like in the diagram below, while still keeping b and c centrally aligned?

Code:

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning,calc}
\begin{document}
\begin{tikzpicture}[auto,node distance=2cm, align=center,
block/.style={rectangle,draw, thick,inner sep=2pt,minimum size=10mm},
paths/.style={->, very thick, >=stealth'},
curve/.style={<->, thick, >=stealth', bend left=270, looseness=2, distance = 60mm},
]

%Draw, place, and label variables
\node [block] (a) {a};
\node [block] (b) [above right=of a] {bbbbbbbbbb};
\node [block] (c) [below= 4cm of b.south, anchor = north] {ccc};
%\node [block] (c) [below right= of a] {ccc}; % ideal curved path between b and c here, but block not centered

% Draw paths and label them
\draw [paths] (a) to node {x} (b);
\draw [paths] (a) to node [swap] {y} (c);
\draw [curve] (b) to node [swap,midway] {z} (c);
\end{tikzpicture}
\end{document}


The following example first places the node z for "z" to the left of node a to get "z" at the same base line as "a". Then the curve is split into two parts, from node b.west to z.east and z.east to c.west. The components .west and .east ensure the correct starting and ending points. The start and end angles of the curve parts are specified by options in and out for path operator to. This requires library topaths:

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning,calc,topaths}
\begin{document}
\begin{tikzpicture}[auto,node distance=2cm, align=center,
block/.style={rectangle,draw, thick,inner sep=2pt,minimum size=10mm},
paths/.style={->, very thick, >=stealth'},
curve/.style={<->, thick, >=stealth'},
]

%Draw, place, and label variables
\node [block] (a) {a};
\node [block] (b) [above right=of a] {bbbbbbbbbb};
\node [block] (c) [below= 4cm of b.south, anchor = north] {ccc};
%\node [block] (c) [below right= of a] {ccc}; % ideal curved path between b
%and c here, but block not centered

% Draw paths and label them
\draw [paths] (a) to node {x} (b);
\draw [paths] (a) to node [swap] {y} (c);
\node [xshift=-3em, anchor=base east] (z) at (a.base west) {z};
\draw [curve] (b.west) to[out=180, in=90] (z.east)
to[out=-90, in=180] (c.west);
\end{tikzpicture}
\end{document}


And a variant, where the straight connections (a) -- (b) and (a) -- (c) are connected at the corners. In this case it might look better.

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning,calc,topaths}
\begin{document}
\begin{tikzpicture}[auto,node distance=2cm, align=center,
block/.style={rectangle,draw, thick,inner sep=2pt,minimum size=10mm},
paths/.style={->, very thick, >=stealth'},
curve/.style={<->, thick, >=stealth'},
]

%Draw, place, and label variables
\node [block] (a) {a};
\node [block] (b) [above right=of a] {bbbbbbbbbb};
\node [block] (c) [below= 4cm of b.south, anchor = north] {ccc};
%\node [block] (c) [below right= of a] {ccc}; % ideal curved path between b
%and c here, but block not centered

% Draw paths and label them
\draw [
paths,
shorten <=-.7\pgflinewidth,
] (a.north east) to node {x} (b.south west);
\draw [
paths,
shorten <=-.7\pgflinewidth,
] (a.south east) to node [swap] {y} (c.north west);
\node [xshift=-3em, anchor=base east] (z) at (a.base west) {z};
\draw [curve] (b.west) to[out=180, in=90] (z.east)
to[out=-90, in=180] (c.west);
\end{tikzpicture}
\end{document}


• Thanks for adding the more aesthetically pleasing version! – luser Apr 22 '15 at 10:35