Could you attach what you're working with? I'm not sure I understand the question. You're looking for a percent of Total on 'Cell' which would yield 100% every time, using the cross tab you've attached. Are you looking for percent of total (of each widget) for the year?
This is the issue / question.
Notice the "blocky" dimensions. This is caused by the absence of a record for those years (as in the sample above).
I can use ZN(lookup(sum([Sales]),0)) in place of [Amount] to replace the nulls with zeros to "smooth" the area chart. But, the ZN calculation does not work for the percent of total area chart. There is not option for a quick table calculation. So, how do I get around this. I finally figured out that I simply need to filter out the null values to "smooth" the area chart.