It would be correct to write
Folglich ist in Abbildung 1 ein Subsystem und in Abbildung 2 sind vier
its simply a convention to leave it out, as it doesnt change the sense.
Folglich ist in Abbildung 1 ein Subsystem dargestellt und in Abbildung 2 sind vier
To leave out redundant terms/symbols like these is also quite common in mathematics. I wouldnt say this a special property of german language. You can translate both blockquoted sentences literally into English. Wouldnt call this a matter of grammar at all...
So your first version would be correct. Think mathematical, what do you want to express, where do you loose sense. From a set theory point of view, ist is wrong as you have several systems.
Of course too much use of redundant words always is boring and no good style, again thats why it is also very common in mathematics. You want to write something down as short as possible to be efficient and gather attention. But redundance is no mistake per se.
Concerning the comments and Oregons reasoning. I think Stefan Walters example is false as he stated. The important factor besides the number of objects is the Syntax, basically it are TWO clauses linked by und
Folglich ist in Abbildung 1 ein Subsystem dargestellt. In Abbildung 2 ist (auch) ein
By inking these two sentences the convention is to leave out all redundant terms in the first clause and draw sind because of reasons of syntax to the front.
Folglich (ist)sind in Abbildung 1 ein Subsystem (dargestellt) und in Abbildung
2 (sind) vier Subsysteme dargestellt
Thats imho the logic behind this convention. Maybe both are allowed, Oregon gave no official rule out, but as i stated above this convention seem to be more influenced by reducing redundance and regarding set theory. This explains to me why the Singular actually not only seems to sound better.