Open source options include Superset and CKAN. As far as I know, you would need to host these yourself and they do require a fair amount of technical knowledge to set up. That's often the tradeoff with open source! You would need to either also host Aggregate yourself so you can access the data or set up a pipeline from your Aggregate instance to your self-hosted visualization system.
For a simpler and free but not open source solution, I've used the Google Sheets streamer in Aggregate and built visualizations there. With pivot tables, it's possible to do pretty advanced analysis.
As @Paul_Storry says, Google Data Studio is a great option for more sophisticated analysis. If you're hosting Aggregate on App Engine, you can go to Google Sheets and then to Data Studio from there.
Other commercial options (with free tiers) include Tableau and PowerBI. With Aggregate, you could set up a near-real-time pipeline by exporting a CSV periodically using the Briefcase command line interface and analyzing that CSV. If your data is public, you can use Tableau public. ODK Central makes it easy to integrate with either of those options in real-time with the OData connector.