Democratic deliberation is taking place nowadays in the digital sphere: discussions in the social media, in the new eDemocracy platforms and in the official parliamentary sessions that create digital data. These deliberation data hold immense potential for analysis and insight, and the interest grows when diverse sources are interconnected. This paper introduces the Deliberation Knowledge Graph, a technological solution to integrate deliberation processes, arguments and participants across different institutional and civic spheres. First, the Deliberation Ontology is presented, a joint data model. Then, the systematic integration of deliberation data from European Parliament proceedings, civic participation platforms, and public forums is described. As an example of application that can have this new technology, the paper describes how this knowledge graph particularly enhances the capacity to examine argument quality, identify reasoning patterns, and trace the evolution of policy positions across different deliberative spaces. Potential applications and problems are discussed.