Create the s curve based on dates

Dear Seniors,

I am working on Project Planning, I need to create the Progress curve for our Project based on Weight Factor every month.

I need to create the Plan and actual curve based on the dates defined in particular activities. Once the activity progress is calculated every month based on cut off date then it will sum product with corresponding WF to get the progress achieved every month. Presently I am calculating the values every month and pasting the values in Chart data every month. I had created the S Curve and attached here.

Now my objective is to automatically calculate the values for the entire cut off date at one time (From Project Start to End). i.e I don't want to do any manual work in the Chart. I will update only the actual Dates in the file, then the curves need to get automatically updated.

1) I don't want to calculate the progress for individual activities, I would like to calculate the overall progress directly and I need your help to do the same.

2) When I change the Cut off date, then the previous values needs to become static.

I hope to get your support to solve my problem soon.

Thanks and Regards
R. Vadivelan