My approach would be to simply have the 8051 do the logging - then upload the data to a PC, and let the PC do all the caluculations & presentation of the analysis.
PCs are much better at analysis & presentation.


That is a bad idea - you really do not want to work with anything connected directly to live mains voltages if you can avoid it!

It is also unnecessary: you already have an isolated, low-voltage signal that's directly proportional to the mains voltage - it is the output of the transformer that powers your circuit!