1. A computer implemented system that facilitates a declarative approach to constructing richly customizable and interactive data visualizations, comprising: at least one processor and at least one memory, the at least one memory storing one or more components executable by the one or more processors, the one or more components including: a receiving component that receives data that conforms to an extensible declarative language, the language includes at least: a set of source descriptors each of which declaratively describes a respective data source, a set of transform descriptors each of which declaratively describes a respective operation applied to the respective data source to yield a transformed result, and a set of visualization descriptors each of which declaratively describes a respective visual representation of the transformed result, each visualization descriptor from the data declaratively describing an appearance, a behavior, or one or more features of the respective visual representation; and a rendering component that reads a visualization descriptor from the data and that composes an associated visual representation of the transformed result in a visualization canvas.