You could assign a sort order to each category in your pie chart, then sort on that instead of the value. Of course, if next month, the values change enough that two different categories are small and close to each other, that might not end up being the best long-term solution.
I'd probably go with a bar or column chart. Pie charts aren't the most visually accurate ways to display data in general anyway. Our eyes have a hard time distinguishing between, say 18% and 20% on a pie chart, whereas with a bar/column chart, you can visually see that one is bigger than the other.