I have 2 sheets called Sheet-A and Sheet-B both containing numeric data
where (the same) rows and columns are only partly filled. Sheet-B also
contains a chart using data from several columns.

The cells of Sheet-B contain formulas referring to Sheet-A like
=IF(ISBLANK(Sheet-A!D31),
Sheet-A!D31,IF($A7<$C$3,Sheet-A!D31-($C$3-$A7),Sheet-A!D31))
The intention is that if a cell a Sheet-A being empty the corresponding
cell of Sheet-B also will be empty and the chart will NOT use the value
0 for the cell but skip it (and draw a line between the previous and teh
next EXISTING value).
The formula above doesn't achieve what i want (I tried the various
possibilities of Tools | Options | Chart and also Tools | Options | View
| Zero values) and indeed cells that SHOULD be blank aren't (tested with
the =ISBLANK(E8) function)

How can I achieve my goal