First off, thank you for developing such a great tool. MongoDB would be crazy not to integrate this into their product. What you're doing seems more robust than IOPro: https://store.continuum.io/cshop/iopro/
Collections can have an array as a field and I believe the only way I can bring that into my Python environment is as type "Object". Currently Mongo offers no way to calculate variance with the aggregate pipeline so I simply $push to an array and then import the dictionary results into Python via PyMongo to calculate the variance of the array for the given field. It would be excellent if we could bring in a field as an array of arrays or object.