The massive amount and variety of city-related data raise equally big challenges to enable citizens to make sense of such data for improving their daily life and fostering collective decision making. The existing dashboards include limited pre-defined use cases which can only address the most common needs of citizens, but do not allow for personalization. In this work, we propose an open source environment that enables citizens to easily explore city-related data. A backend continuously collects heterogeneous data, turns them into a unified format and exposes them through an API; a front-end allows users to create interactive visualizations combining different data sources and visual models, and to share them to engage others. In this way, citizens can build up a data-driven public awareness supporting an open, transparent, and collaborative city.