Mongo is good for store-and-retrieve, and aggregation along a single dimension, and not as strong at the operations mentioned above.
Have you benchmarked Mongo against a relational db for analytic operations on a multidimensional model? The results could be interesting and perhaps different from what you would expect.[1]
Many new OLAP products are implemented on pure relational databases for performance reasons. Some databases with columnar indices are even faster for OLAP type operations.
[1] That said, Mongo could be a good choice for latency reasons. If your spreadsheet is doing lots of small calculations, then it makes sense to use something that can return results quickly.
Nice work though! Keen to test it out. Hopefully a mobile version is on the roadmap.