Abstract: In the development process of powertrain systems, 2D function ensembles frequently occur in the context of multi-run simulations. An analysis has many facets, including distributions of extracted features, comparisons between ensemble members and target functions, and details-on-demand. The primary contribution of this paper is a design study of an interactive approach for a comparative visual analysis of 2D function ensembles. The design focuses on a tight integration of domain-oriented and member-oriented visualization techniques, and it seeks to preserve the mental model of 2D functions on multiple levels of detail. In this context, we propose a novel focus+context approach for visualizations relying on data-driven placement which is based on labeling. We also extend work on feature-preserving downsampling of 2D functions. Our design supports a comparison of 2D functions based on juxtaposition, overlay, and explicit differences. It also enables an analysis in terms of extracted scalar features and 1D aggregations. An evaluation illustrates a workflow in our application context. User feedback indicates a time saving of 70% for common tasks and a qualitative gain for the entire development process.