Defining a Simple Sequence Diagram

A sequence diagram is defined in three main phases:

Object definition
Message exchange
Object lifeline completion

The following is an example of a very simple UMLGraph sequence diagram (from now on we will ommit the .PS/.PE and copy "sequence.pic"; elements.)

.PS
copy "sequence.pic";

# Object definition
object(S,"s:Switch");
object(P,"p:Pump");

# Message exchange
message(S,P,"run()");
message(S,P,"stop()");

# Object lifeline completion
complete(S);
complete(P);

.PE

The above code, defines two objects, S and P, labeled as "s:Switch" and "p:Pump". Objects are placed in the diagram from left to right, in the order they are defined. All defined objects are initially inactive.

The code then sends a messages from S to P labeled "run()", and another one labeled "stop()". Each message automatically advances the sequence diagram timeline by a single step.

Finally, the code completes the lifelines of the two objects.

The resultant diagram is
Simple sequence diagram