We present the metric lens, a new visualization of methodlevel code metrics atop UML class diagrams, which allows performing metric-metric and metric-structure correlations on large diagrams. We demonstrate an interactive visualization tool in which users can quickly specify a wide palette of analyses, based on color-mapping, scaling, and sorting metric tables on UML diagrams. We illustrate our technique and tool by a sample complexity assessment analysis of a real-world C++ software system.