Whether Portia is on stage or not, she is central to every important action in the play. Do you agree?
You know, it's funny. I just set off answering that question and as I wrote my answer arguing that she wasn't central, I realised that actually she is quite central.
It all goes back to a conversation Antonio and Bassanio have in Act 1, Scene 1, right at the top of the play. Bassanio wants to go to Belmont to see Portia and try and win her hand in marriage. Antonio replies:
Thou know'st that all my fortunes are at sea;
Neither have I money, nor commodity
To raise a present sum: therefore go forth,
Try what my credit can in Venice do;
That shall be rack'd, even to the uttermost,
To furnish thee to Belmont, to fair Portia.
So the reason that Antonio gets into the "pound of flesh" bond with Shylock is because of Portia - because the money it provides allows Bassanio to go to Belmont to win her.
This means that, when Antonio's ships are lost and he must pay the bond, it is still, really an event which has its root with Portia. The trial scene too, not simply because Portia is there to save the day at the end, all comes from that initial bond that Shylock made with Antonio, and so is also Portia-centric.
Obviously, the key events in Belmont with the caskets and the resulting marriages are all centred around Portia.
The only exception I can really think of Jessica's elopement with Lorenzo, and her escape from Shylock's house. That doesn't really have anything to do with Portia - though they do go to Belmont.
So are all the events Portia-centric? Nearly, yes.