Most theories of graph comprehension posit the existence of a graph schema to account for people's prior knowledge of how to understand different graph types. The graph schema is, however, a purely theoretical construct: No empirical studies have explicitly examined the nature of the graph schema. We sought to determine whether graph schemas are based on perceptual features or on a common invariant structure shared between certain graph types. We isolated the process of activating the graph schema by presenting the graphs to participants in pure and mixed blocks. Any differences in reaction time between the blocks could be attributed to loading the appropriate schema. Results from a series of experiments using five types of graphs suggest that graph schemas are based on the graphical framework, a common invariant structure among certain types of graphs. These results provide insight into the comprehension of novel graphs.