Continuing with yesterday, today I’ll be talking about how to draw graphs in latex. The package I like to use is tikz. In my last post “Drawing Trees in Latex,” I discussed relative positioning and it didn’t make much sense in terms of trees, however, it is more relevant when drawing graphs as you will see in a moment. As we all know, a graphs consists of two elements:

the set of vertices
the set of edges

Vertices

Here is an example with how to declare vertices with relative positioning in tikz:

\begin{tikzpicture}
\node (A) {A};
\node (B) [left =of A] {A};
\node (C) [below =of B] {C};
\end{tikzpicture}

Here we declare three nodes A, B, and C. Every node declaration should end with a semicolon or you will get a compile error. It was different when drawing trees because starting from the root node was one whole node declartion. Rule of thumb, semicolon for each \ .

Edges

Undirected

Here is an example with how to draw edges, and I’m sure there are other ways to draw this, but the way I’m going to show is the most intuitive to me.

\begin{tikzpicture}
% nodes from previous example
\node (A) {A};
\node (B) [left =of A] {A};
\node (C) [below =of B] {C};

% edges
\path
(A) edge (B)
(B) edge (C)
(C) edge (B);
\end{tikzpicture}

It is important that you put a semicolon after the last defined edge in the path. But basically you use the names of the nodes and tell tikz to draw and edge between them with, you’ve guessed it, the keyword edge. It’s really just that simple.

Directed

To draw directed graphs with tikz is really simple first you must tells tikz to use the arrow library by stating \usetikzlibrary{arrows} . If you are using another library, seperate them by a comma. You just change the declaration of a tikzpicture to \begin{tikzpicture}[->] . That arrow will tell tikz to use arrows for edges. But depending on your preferences, the arrowhead may not be to your liking, you can change it to the latex default arrowhead by adding \tikzset[>=latex] before you begin a tikzpicture.

Reversing a graph

To reverse a graph is really simple as well. When you specified -> when declaring a tikzpicture, all you have to do is flip the direction of the arrow, and the arrows will now point in the opposite direction.

Graph edge labels

Before we talked about how to create edge labels for trees, and in this case the syntax is not so different. For edge labels, there are positioning keywords that you can use to place the label:

above
below

Here’s an example of this in action:

\begin{tikzpicture}
% nodes from previous example
\node (A) {A};
\node (B) [left =of A] {A};
\node (C) [below =of B] {C};

% edges
\path
(A) edge node[above]{edge ab} (B)
(B) edge node[below]{edge bc} (C)
(C) edge node[below, show=none]{edge cb} (B);
\end{tikzpicture}

So basically for edge labels append node[settings]{label} between edge and the end vertex of the edge.