Problem with variable as dimension - bar chart

[Anonymous]

Hi,

i try to do Trellis Chart showing month for specific Unit but showing only 2 units from 5.

List of My Values/Units= [Jednostka Bizensowa]

JB01, JB02, JB98, JB97, JB00

My Expression is like this:

If(left([Jednostka biznesowa],4)='JB01' or left([Jednostka biznesowa],4)='JB02', left([Jednostka biznesowa],4),null())

In this way when i have not had selected any values (and i want to leave this in that way)  i have something like below:

This strange "-" is my null (). Is is possible to write expression to not show this null value, only JB01 and JB02?

Thnak you in advance for your support,

Jacek Antek