To the extent a diagram like that is showing pleasure and pain to be on an opposite axis then it would be consistent and helpful to explain that the presence of one is the absence of the other.
However to the extent that the diagram implies that "excitement" is on a scale of its own that is unrelated to describing pleasure and pain, it probably detracts from the ultimate philosophical point that if a person is conscious and aware at all, he is feeling either pleasure or pain with no zero / neutral point.
Of course if you follow the descriptions around the circle the point is to read them together and you don't get confused.
I'd say that there's not only no 0,0 point, but none of the points at all marked by the vertical line. If the two lines are "X" and "Y" there's no 0,Y point up and down the circle.
This aligns with my intuition. But it's difficult to intuit the idea that the absence of pain is the greatest pleasure. It’s logically correct, but when I really dig into it, it's hard to wrap my head around. At first it makes sense at the macro level, but Eoghan Gardiner 's anecdote is at the macro level. After a while it makes my head spin!
Of course, what Cicero loved to do was to find rabbit holes and dig away...
And yep this is the issue, but (1) there is good reason to believe that the Torquatus portion was coming straight from Epicurean textbooks, and (2) it's essential to point out that the criticism that this perspective is a rabbit hole is the Ciceronian anti-Epicurean position.
It seems to me the key decision that every reader has to make is to choose from one of these two positions. Either: (1) Torquatus' formulation is a thorough misstatement of Epicurean philosophy, or (2) Torquatus' formulation is correct but appears foreign to our ears because Cicero does not allow Torquatus to include the full explanation of how "absence of pain is the greatest pleasure" arises from a necessary logical deduction, given the inverse relationship of pleasure and pain.
Somebody's going to do a better job than my chart here, but hammering this point is going to be necessary to stop one's head from spinning on what Torquatus is saying.
Words mean something. When you accept (1) that if you are aware of anything at all then what you are aware of is either pleasure or pain, and (2) someone has said that their life is "free from pain," then (3) the realization that what they have said is that they are at maximum pleasure follows like night follows day.
In Eoghan's case I see him following Cicero's wording, which I would discourage, if the implication is that pleasure is associated with excitement / stimulation alone. The wider perspective is that excitement / stimulation is not a required component of pleasure, so it is an incorrect deduction to hold that the level of excitement/stimulation has any relationship to the total percentage of pleasure being experienced.