I used to use a sequential diagram for a process flow between multiple components, but have never seen it in patent documents. Is it not allowed?
I agree Eric Shain. In fact, this kind of diagrams are quite popular en communication protocol related patents, where interactions between sender and receiver have a sequential order. For example, figures 3, 4 & 5 in Amazon patent US6675196; figure 2 in ATEN patent US7716404B2; or figure 2 in patent application US2004/0100945 A1.
Sometimes, you can even find directed graphs with multiple paths to show the flow of communication, as figures 2, 3a, 3b, 4a, 4b in Hughes Aircraft Company patent US5485464.
Diagrams, illustrations, photos and graphs are all allowed in a patent application. They are referred to as drawings under 35 U.S.C 113:
The applicant shall furnish a drawing where necessary for the understanding of the subject matter sought to be patented.
Note from the U.S.C clause above that the purpose of any drawing is only to facilitate the understanding of the subject matter that one is trying to patent.
Within the context of an utility patent, only the Claims are protected under the U.S. Patent Act.
Therefore, if the said diagram illustrates the method/process that one is trying to patent, the sequential steps of the diagram must be incorporated in the Claims themselves.
FiG.2 of US20120287823A1 shows a similar diagram.
I would expect that a drawing that conforms to USPTO drawing requirements (black and white comes to mind, exceptions for color is accepted only under documented conditions). A search of patents.google.com would produce the examples provided by user=David.
If you suspect that something is prohibited, searching the relevant USPTO requirements (in this case drawings) is the 'do it yourself' method of determining eligibility. One has to be careful using precedence in the sense that the rules are changing: in theory changes in rules will be documented in the updated requirements. This response is not to discourage one from answering the question. It is intended to generalize the process and approach solving this class of problem.