Planar st-graphs are used in a number of different application fields in the sciences, but also in industry. So far, mainly node-link-based layouts have been used to visualize such graphs especially in the Graph Drawing community. One drawback of these standard layouts is their high consumption of space. In Information Visualization, there exist visualization techniques for graphs which achieve considerable space savings, such as matrix-based approaches. In this work, we present a novel space-filling representation to visualize planar st-graphs.