It actually somewhat depends on what you describe the current state of your system as.
If you are yet to start working on your system/it will be developed in future, you can use phrases like-
For the system that will be developed, the diagram is shown below.
The diagram, for the system that will be developed, is shown below.
If you want to indicate that you are already working on your system(work-in-progress), consider using-
For the system being developed, the diagram is shown below.
There are some other phrases that can be used as well, depending on the conjugation of the verb "develop" with various tenses. Check out this link for a detailed reference.