1. A computer implemented system that facilitates a declarative approach to constructing richly customizable and interactive data visualizations, comprising: 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; 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.