fix issue where pie strokes disappear after dataset update

[ztsai]

example: https://jsfiddle.net/12Lmx4jh/2/

[palantirtech]

Thanks for your interest in palantir/plottable, @ztsai! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[ztsai]

Hi @themadcreator, I don't think I am authorized to assign reviewers