Note that if the binning process results in a phase bin with less than 2 occupants, then that bin is ignored in the chi- squared calculation. This is because no variance for that point is available.
The option for weighted or non-weighted mean is included because whilst a weighted mean will give smaller errors, it will also tend to drag the value of each phase bin down if the errors are poissonian because of the way the small variances are associated with low count rates, somewhat artificially.