Overview of directed graph editing

digraph G { "a" -> "b"; "a" -> "c"; { rank = same; "b"; "c"; } }

When entering directed graphs in TWiki, everything you do is to define nodes and their edges (or connections); the directed graph engine then does all placing by itself, fully automatic.

No layout more than which nodes and their relationship, together with node and edge style is defined by the author; the engine takes care of all layout and placing of nodes, which makes creating advanced graphs extremely simple and fast.

Using more advanced syntax very advanced graphs can be created, including subgraphs and balanced trees and record graphs. This document only focusses on "normal" directed graphs; if you want to experiment more with the graph engine, consult the dotguide on www.graphviz.org.

digraph G { a -> b; }

<dot>
digraph G {
 a -> b;
}
</dot>

In order to describe a graph you start by typing:

<dot>
digraph G {

}
</dot>

Inside those two parts you place your graph description. You can define nodes and edges (connection between nodes); you define edges by typing "node1" -> "node2"; which also implicit defines both nodes node1 and node2. If the nodes' names doesn't contain spaces or other special characters, the quotation marks can be ignored.

As you can see in this example we actually only define exactly one edge, the link between node a and node b, by typing a -> b;. Note also how the line is terminated by a semi-colon (;).

digraph G { a [style=filled, color=green]; b [style=filled, color=red];

a -> b [style=dotted, color=blue]; }

<dot>
digraph G {
 a [style=filled, color=green];
 b [style=filled, color=red];

 a -> b [style=dotted, color=blue];
}
</dot>

This example is very similar to the above example, with the difference that we have defined some attributes to both the nodes and the edge. As you can see the nodes are filled with green and red, respective. The edge has been changed to have a dotted line instead of a solid line.

If you want to assign attributes to nodes, the nodes have to be defined first, which means they appear on a line on their own. The order of node and edge definitions is not important, the whole file is parsed before any layout is begun.

The first line in the graph

a [style=filled, color=green];

style=filled

color=green

digraph G { a -> b [label="link"]; }

<dot>
digraph G {
 a -> b [label="link"];
}
</dot>

Edges can be labeled by adding the attribute

label="text"

digraph G { a -> b; a -> c;

subgraph cluster0 { style=filled; color=lightgrey; rank=same;

node [style=filled, color=white]; b; c; }

c -> d; b -> d; }

<dot>
digraph G {
  a -> b;
  a -> c;

  subgraph cluster0 {
    node [style=filled, color=white];
    style=filled;
    color=lightgrey;
    rank=same; 
    b;
    c;
  }

  c -> d;
  b -> d;
}
</dot>

If you want to group nodes together, you put the node definitions inside a subgraph clusterXXX { } statement.

Note: the name of the subgraph must begin with cluster.

The subgraph has the attributes

style=filled

color=lightgrey

rank=same

You can also see the nodes b and c are defined in the subgraph by the last two lines in the subgraph.

All nodes defined in the subgraph have the attributes

style=filled, color=white

digraph G { size="2.5,2";

{ node [shape=plaintext, fontsize=16];

1975 -> 1980 -> 1985; }

{ rank=same; 1975; a; b; c; } { rank=same; 1980; d; e; } { rank=same; 1985; f; g; h; i; }

a -> d; b -> e; c -> e; d -> f; e -> g; e -> h; e -> i; }

<dot>

digraph G {
  {
    node [shape=plaintext, fontsize=16];

    1975 -> 1980 -> 1985;
  }

  { rank=same; 1975; a; b; c; }
  { rank=same; 1980; d; e; }
  { rank=same; 1985; f; g; h; i; }

  a -> d;
  b -> e;
  c -> e;
  d -> f;
  e -> g;
  e -> h;
  e -> i;
}
</dot>

In this graph three groups are defined, by putting them inside a block, defined by { }.

The first group defines the nodes 1975, 1980 and 1985, by connecting them together; these nodes are of the type "plaintext" with font size equal to 16. We do this because the years are to be placed to the left as a timeline and therefore the font size is choosen to be a little larger.

The next three groups for example

{ rank=same; 1975; a; b; }

Following the last group definition we define all edges.