Thanks Jim. I guess i didn´t explain myself well. There is no missing data in the table. It just happens, that for one charge all the wafers have meausrments (data) while another charge only has two wafers (or consist of two wafers). But in general there are 49 wafers (nominal) per charge (nominal).
When i set wafer as the wrapping factor, there will always be the full number of wafers for each charge as cells - even if there is no real wafer for that charge.
Sorry that i depict it so confusing. I meanwhile built a workaround for me anyways by generating a string out of charge and wafer. Hence every measurement is unique.