[Tex/LaTex] Accessing the middle of a path


In the following example, I'd like to move the dotted vertical lines to the (horizontal) center of the arrows connecting the lower elements. For example, the right dotted line should be centered with regard to the arrow connecting l2 and l3. The best I could do is centering it between the nodes l2 and l3, which looks bad since the arrows are of different length.

enter image description here




\begin{tikzpicture}[x=2cm, every path/.style={>=latex'}]

\node (h1) at (.5, 1) {$\{S1,S2\}$};
\node (h2) at (2, 1) {$\{S4\}$};
\node (l1) at (0, 0) {$\{S5, S6\}$};
\node (l2) at (1, 0) {$\{S5, S7\}$};
\node (l3) at (2, 0) {$\{S4\}$};    

\draw[->] (-.75, 1) -- (h1);
\draw[->] (h1) -- (h2);
\draw[->] (-.75, 0) -- (l1);
\draw[->] (l1) -- (l2);
\draw[->] (l2) -- (l3);

\draw[dotted] ($ (l2) !.5! (l3) $) to ++(90:1);
\draw[dotted] ($ (-.75,0) !.5! (l1) $) to ++(90:1);


Best Answer

In the example you give, the line is drawn at the midpoint between the centers of nodes l2 and l3. One solution is to add a coordinate at the midpoint of the lower line between l2 and l3, call it l4

\draw[->] (l2) -- (l3) coordinate[midway](l4);
\draw[dotted] (l4) to ++(90:1);

gives me a line halfway along the arrow between {S5,S7} and {S4}. enter image description here