Stacked gantt bar chart - x-axes value changes when adding expressions

[#1]

Hi!

I have a horizontal bar chart, with project phase dates. When only expressions that affect a certain project are abled, the x-axes is showind dates correctly. 

But when I able expressions that do not concern that particular project (aka returns null) the x-axes values change to numbers.

The bars are still "correct" ones but the count is messing up somehow I think. What is causing this?

Dates are coming from these fields.

I have expression for each step like

RangeMax(above(Max ([Opportunity Identification]),0,rowno())) // Upper limit
-
RangeMax(Min ([Added Date]),RangeMax(above(Max ([Opportunity Identification]),1,rowno()-1))) // Lower limit .

So there is few dozen expressions. The above expressions returns null when either of the fields is empty. I image the reason is in the expressions in some how but I don't know how to fix it. Can I use the alt() function or some other function to make the expression not count when either of the fields is empty for example? Or is the solutions even more simple? My question is, how to make the chart do the calculations so that the end result is showing the x-axes in dates, not numbers?

I have unchecked the "forced 0" for all expressions, bar offset is min(Date) for some of the expressions - not all. And all expressions have scale static min and static max according to the min([Added Date])-10 and max([Delivery Month])+200.

Any idea what should I try?

Thanks for your help!

Reg, Heidi