The development and adoption of Personal Data Management Systems (PDMS) have been fueled by technical means, privacy regulations, and smart disclosure initiatives. A PDMS makes it easier for users to collect, process, and share personal data. However, functionalities based on collective computations within a network of PDMSs are still lacking at least in commercial products. This demonstration bridges this gap by leveraging the open-source Cozy Cloud product and recent research results in the area of privacy-preserving decentralized data aggregation. Our demonstration scenario highlights both the utility aspect of collective computations and the main features of the aggregation protocol.