Hi @jthomas ,
From what I understand of your question, “flexible reports” are dhis2 analytics with dynamic filtering and grouping, allowing for greater interaction. In particular you’re interested in exploratory analysis of tracker data.
I can recommend you read through the Standard Reports functionality of DHIS2, which allow you to create reports in HTML and Javascript. Note that the D3 and NVD3 libraries are added to dhis-web-commons-resources so you might be able to build some interactive charts in the report with custom Javascript.
If you want to build program indicator disaggregations more efficiently, you can try the Program Indicator Disaggregator app from HISP-Tanzania. It is available on Github here, and recently won the DHIS2 Apps Competition 
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Nice if you could clarify the question if I am mistaken, or share more details of your requirements… If there is a specific feature in mind, feel free to open a feature request on Jira.