The opposite of implausible is plausible.
The word that belongs on the right-most side of your plausibility scale is convincing.
I would tend to disagree about the relationship you have described between implausible/plausible and improbable/probable.
improbable: "It is unlikely."
probable: "It is likely."
implausible: "I cannot imagine that there could be a case in which this would be true."
plausible: "I can imagine that there could be a case in which this could be true."
(im)plausible is based upon belief and opinion, whereas (im)probable is based upon statistical fact. The level of plausibility is based strictly upon the depth of knowledge on the subject, gullibility, and/or faith of person providing the opinion. To someone, a thing could be implausible while still being certain in reality.
convincing: "I cannot imagine that there could be a case in which this would be false."
Convincing does not imply a certain outcome, but it does imply an individual's certainty by belief. Again, because you're trying to compare a subjective measure with a concrete measure, there will be oddities in definition. For example, a statistical probability that is certain is guaranteed to happen 100% of the time. Conversely, an individual can be certain about an outcome, yet still be proven incorrect.
An individual that is skeptical or closed-minded about a subject could have a plausibility scale measuring similar to this:
|---------------implausible---------------|--plausible--|
|---------improbable---------|---------probable---------|
An individual that is gullible or imaginative about a subject could have a plausibility scale measuring similar to this:
|-implausible-|----------------plausible----------------|
|---------improbable---------|---------probable---------|
A person's perception of a subject does not change the probability.