Hacer que la ruta curva parezca paralela en relación con un nodo TikZ

Question

Tengo un diagrama que se parece a este:

Tengo nodos alineados horizontalmente by ccon respecto a sus centros, pero esto causa un problema con el camino curvo zdebido a que ya no es simétrico.

¿Cómo puedo hacer que los extremos del camino zsean paralelos entre sí y que la etiqueta se alinee con el nodo a, como en el diagrama a continuación, sin dejar de bestar calineado centralmente?

Código:

\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}

Answer 1

En el siguiente ejemplo, primero se coloca el nodo zde "z" a la izquierda del nodo apara obtener "z" en la misma línea base que "a". Luego, la curva se divide en dos partes, desde el nodo b.westhacia z.easty z.easthacia c.west. Los componentes .westy .eastaseguran los puntos de inicio y finalización correctos. Los ángulos inicial y final de las partes de la curva se especifican mediante opciones iny outpara el operador de ruta to. Esto requiere biblioteca 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}

Y una variante en la que las conexiones rectas (a) -- (b)y (a) -- (c)se conectan en las esquinas. En este caso podría verse mejor.

\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}