Overview

To express knowledge about visualizations, we first need a language to describe visualizations. Draco describes visualizations and context around them (e.g. properties of the data and task) as facts. In this section, we will describe the kinds of facts we chose for Draco.

Draco has a Generic Specification Format

The constraint solver Clingo reasons about the facts that describe a visualization. The specific format we use to describe facts to the solver is as a function of the name entity or attribute.

Entities

Entities describe objects. The facts entity describes what other entities associate with an entity. If an entity has no parents, we use a special identifier root. For example, entity(view,root,v1). says that there is a view v1 on the root.

Attributes

Attribute facts specify properties of entities. For example, the number of rows in the dataset in this function form is attribute(number_rows,root,42). (read as the number of rows (on the root entity) is 42). Attribute names can be tuples, which allow us to distinguish the same attribute that exists on different entities. For example attribute((field,type),f1,number). specifies the type of a field (here f1) while attribute((mark,type),m1,bar). specifies the mark type (here of m1).

Draco’s Specification Format for Visualizations

Note

You could design your own description language and use it with Draco or extend the existing language we use here. If you add new information, Draco will not immediately use the new facts until you also add constraints over the facts.

Draco uses an encoding based on the Grammar of Graphics (GoG) [Wil12] and Vega-Lite [SMWH16]. While our specification format is inspired by Vega-Lite, Draco is not limited to the features Vega-Lite supports.

At the core, a Draco program encodes visualizations in views. A view can contain one or more marks that encode data and corresponding scales. Besides the visualization, a Draco program can describe the data schema and the primary visualization task. All of these definitions are extensible and you can add whatever other properties you care about to Draco.