I've managed to reproduce the issue you've been having by doing this:
_chart.plotAreaBorderColor = [UIColor whiteColor];
_chart.plotAreaBorderThickness = 2.0f;
Do you have any code like this in your project? If you just don't set the plotAreaBorder thickness (it's default is zero) then the problem should go away.
Let me know if this helps and if you still have issues then please let me know.