Learning from text is a "constructive" activity in which sentence-level information is combined by the reader to build coherent mental models. With increasingly complex texts, forming a mental model becomes challenging due to a lack of background knowledge, and limits in working memory and attention. To address this, we are taught "knowledge externalization" strategies such as active reading and diagramming. Unfortunately, paper-and-pencil approaches may not always be appropriate, and software solutions create friction through difficult input modalities, limited workflow support, and barriers between reading and diagramming. For all but the simplest text, building coherent diagrams can be tedious and difficult. We propose "Active Diagramming," an approach extending familiar active reading strategies to the task of diagram construction. Our prototype, texSketch, combines pen-and-ink interactions with natural language processing to reduce the cost of producing diagrams while maintaining the cognitive effort necessary for comprehension. Our user study finds that readers can effectively create diagrams without disrupting reading.