Well, I thought all it would take is setting up the [Accumulated Sales] column so that it was null when ABC was null without referring to the ABC column. There might be an easier way, but this seems to work:
<= (A_Perc + B_Perc + C_Perc) * Sum(Total MonthlySalesNetValue)
and (rowno()=1 or above([Accumulated Sales]))
However, even when that is the ONLY expression in the chart, and we're suppressing nulls, the null rows do not suppress. I honestly don't know why.
Anyway, if they DID suppress, then all the other expressions could just take this form to make them suppress as well.
Maybe it's a starting point. I'm probably missing something obvious about why the rows are suppressing, so we could be just one simple step from an answer.