|There really is no way to use a formula to create an "empty" cell since the cell will contain the formula.|
Here are 2 methods to create a graph that will ignore the blank cells:
1 - This one I've used in the past:
Create an AutoFilter for your data and select "NonBlanks". Only the visible cells will be graphed.
2 - This one I stole without permission from here and then modified:
Let's say your data resides in A1:A10 and a few of those cells are blank due to your formula.
Enter this in B1 and drag it down to B10:
Enter this in C1 and drag it down to C10: (formula split to fit in this post)
Column C should now contain a contiguous list of the non-blank values from A1:A10.
Graph the values in Column C.
BTW, instead of this...
why not use this?
=IF(B121="", "", B121-6000)