Information visualization tools have traditionally implemented a set of pre-defined visual displays. We describe the DOODLE Visualization Tool, which is interactive and supports visualizations specified by the user with a visual constraint-based language. The main modules of the tool comprise the syntax-directed user interface, the parser for the user's specification, and the constraint solver. The strengths of our approach include the expressiveness of the visual language, the efficiency of the constraint solver, and the overall flexibility and extensibility of the framework. The user interface is implemented using Java and is available on the WWW.