If you want to improve readability, I suggest removing clutter (so called chart junk) first.
If you have all the data points labelled, there's no need for the vertical axis labels. But that's minor. It'd be much cleaner the other way around, without the numerous labels — people supposedly know how to read charts. Either way many ticks could go.
If the labels remain, I suggest at least rounding. I guess the underlying library just sucks and you can't improve placement (overlap, cutoff)? And you could only draw them when the slope changes or there is an intersection.
I'd also suggest a bit thicker connecting lines, so when there's more of them, it's easier to match the colors with the legend, since it's more likely similar ones will get used.
The legend could also use more spacing. Personally I prefer vertical ones, since all the markers are then aligned and easier to compare.
As for the baseline: like I said, I'm not sure what the best would be. Perhaps 0 (for relational integrity), but perhaps not, it's combat after all. So maybe fists at STR 24, which could have respectable damage in theory, but I guess crappy APR and to hit chances that likely lead back to 0.