Recent graphical interfaces offer direct manipulation mechanisms for authoring visualizations, but are largely restricted to static output. To author interactive visualizations, users must instead turn to textual specification; but, such approaches impose a higher technical burden than their graphical counterparts. To bridge this gap, we introduce interaction design by demonstration: a novel method for authoring interaction techniques via direct manipulation. Users perform an interaction (e.g., button clicks, drags, or key presses) directly on the visualization they are editing. The system interprets this performance using a set of heuristics, and produces suggestions of possible interaction designs. Heuristics account for properties of the interaction (e.g., target and event type) as well as the visualization (e.g., mark and scale types, and multiple views). Interaction design suggestions are displayed as thumbnails; users can preview and test these suggestions, iteratively refine them through additional demonstrations, and finally apply and customize them via property inspectors. To evaluate our approach, we instantiate it in Lyra, an existing visualization design environment. We demonstrate its expressive extent with a gallery of diverse examples, and evaluate its usability through a first-use study and via an analysis of its cognitive dimensions. We find that, in Lyra, interaction design by demonstration enables users to rapidly express a wide range of interactive visualizations.