Active Report - Data Properties - What are these and how do they work?!

[Philbo]

Hi all,
I'm relatively new to Cognos development (7 months in!), and have not had any official training, so apologies if I ask any stupid questions!

I'm trying to understand how the Data Properties work in a report object. Or more accurately, why they're not working as I would expect! I'm not talking about properties generally, but specifically the "Properties" option under the "Data" section of the properties tab.

The in-built description reads: "Specifies a list of data items from the query in scope to associate to the selected object. This is necessary when you want to reference a data item that is in the associated query, but not used in the layout."

From that, I assumed I could associate an item as a property, and then use it in the interactive features of my active report.

Specifically, I have a Chart that plots Value by Month. The month is available in the query in 2 fields - Date (i.e. 01/01/2019 in date format) and Month (i.e. 201901 in string format). My chart uses the Date in it's X/Category axis, so I can format it how I wish (Mmm YYYY), but it's the month that I want to pass to an interactive string variable, partially because I have had trouble in the past using date variables.

This all sets up in the report designer fine. I select the category axis Chart Node Member, and am able to assign the Month as a property. I am then able to select the Month property in the Interactive Behaviour screen and use it to set my varMonth string variable. All good so far.

However, when I run my report, I can see that the varMonth variable is not actually being set. I can only get it to set if I include the Month in my chart Axis, which I don't want to do.

Does this functionality just not work, or am I doing something wrong or misunderstanding it's intention?

Any help is greatly appreciated!