We've made changes in XF 1.5 to try to resolve this very specific case, but ultimately this will still happen with larger numbers on that axis due to the differences in how Mac OS X and different browsers render fonts. The root cause is likely some sort of miscalculation happening in the jQuery Flot plugin that we use to display the charts. This may have been fixed in more recent versions but that could potentially be a big change for such a small issue so this is likely the best we'll do for now.
In other news, we have improved the styling here by implementing some responsive design on the charts rather than using fixed widths. This improves the display generally on smaller screen sizes whereas previously it would break out of the container.