A sequence diagram: its participants, left to right, and its messages,
top to bottom.
An object rather than a bare list for the reason GanttData is one:
a participant no message reaches is still a lifeline on the page, and a
list of messages has nowhere to declare it.
The lifelines, in the order the diagram draws them. A participant a
message names that is not declared here is added after them, in the
order the messages first name it.
A sequence diagram: its participants, left to right, and its messages, top to bottom.
An object rather than a bare list for the reason GanttData is one: a participant no message reaches is still a lifeline on the page, and a list of messages has nowhere to declare it.