Clusters of software artifacts that are frequently changed together are subsystem candidates, because one of the main goals of software design is to make changes local. The contribution of this paper is a visualization-based method that supports the identification of such clusters. First, we define the co-change graph as a simple but powerful model of common changes of software artifacts, and describe how to extract the graph from version control repositories. Second, we introduce an energy model for computing force-directed layouts of co-change graphs. The resulting layouts have a well-defined interpretation in terms of the structure of the visualized graph, and clearly reveal groups of frequently co-changed artifacts. We evaluate our method by comparing the layouts for three example projects with authoritative subsystem decompositions.