Custom Aggregation in Apex IR footer

As I was not to much involved in proffi Apex programming since 2008, when I build most valuable Croatian WEB site, Croatian Depository & Clearing Company, where more then 50 bil. € stocks are saved, I didn't have to much challenges in this area until recently.

The problem

Then I got an interesting ask: "In Apex Interactive report, calculate index column as average on two other visible columns and show "Total average". First part was easy-SQL column as average calculation of other two. But problem came when I wanted to show "Total average" value, because I couldn't use implemented IR feature "Aggregation" on average column-average computation on average values is not correct average!!! So I let this value to blank to see what can be done later.
Let me show a picture of problem with report with activated "Control Brake" on first two columns, what results as virtual two reports:

The solution

Analyzing page source with Web Developer show me next data:

 

Hopefully there was created placement for average calculation, but it was blank.
Thanks to Davor Zelić who has enough nerves and will to handle this problem, solution was made in a classic way-custom java script.
The result is this source javascript file which setAggrIndexValue method is the one that do the work, while getElementsByAttribute method is used to find proper values for calculation.
This java script file, as usual, has to be uploaded in workspace/application image repository (or place on Apache images location if using Apache as WEB server), and put it's call directly on custom page header or in page template (find more appropriate solution).
After that on page, where Interactive report is placed, fill call of java script function in page property in "Footer" part of "Header and Footer" section as:

If you start that report and look now in WEB Developer debuger inspector, you'll see that values are filled properly-with real average values.

Result is exactly what I wanted, even thought if there are more then one average calculation on page ("Control Brake" case).

The End

As you see, this solution is pretty generic and real text function call can be viewed as pure text in mentioned java script source, last three lines as comment. The only condition is that calculation values are visible in Interactive report, what is more then common requirement and reasonable condition.
Beside all thisi approach allow very easy implementation of other custom aggregation when you have need for that.
Hope this helps someone.

Cheers!