This paper presents a model of perceptual task effort for use in hypothesizing the message that a bar chart is intended to convey. It presents our rules, based on research by cognitive psychologists, for estimating perceptual task effort, and discusses the results of an eye tracking experiment that demonstrates the validity of our model. These rules comprise a model that captures the relative difficulty that a viewer would have performing one perceptual task versus another on a specific bar chart. The paper outlines the role of our model of relative perceptual task effort in recognizing the intended message of an information graphic. Potential applications of this work include using this message to provide (1) a more complete representation of the content of the document to be used for searching and indexing in digital libraries, and (2) alternative access to the information graphic for visually impaired users or users of low-bandwidth environments.