I'm convinced with the graph except for when x=0.
When $x=0$, the collector is at 0 potential. So photoelectrons that are emitted from the plate are not influenced by any electric field.
Since there is no electric field, the photoelectrons are equally likely to go in a particular direction (towards collector plate) regardless of their energy.
So, since the same amount of electrons are being emitted from the plate by source of different frequencies and since the electrons are equally likely to go towards the collector plate,
I say that at $x=0$, the photocurrent should be the same for all three curves.
Please tell me whether what I have said is right or wrong. And if it's wrong, why is it wrong?
Experiment diagram: !