This looks like a comma or question mark but they're on my voila and dianthus and I don't think those are these cats food of choice.
I believe this is the one and same cat that was on my baptisia in the spring. None of the plants I've found them on are list as foods of choice but it certainly looks like the same cat.
The one in your photo may be large enough to be looking for a place to pupate. Unless you saw it actually feeding on the plant, that might be a possibility.
Looks like a Variegated Fritillary to me. You don't have any passion vine, do you? Or violets, maybe wild ones?
Duh, I just read you said viola; that's a host plant for several types of fritillaries.