The overall goal of our research is a system which can recognize the intended message of a grouped bar chart by reasoning about the communicative signals contained in the graphic. One such communicative signal is the relative effort required to perform different perceptual tasks on the graphic. This paper presents our methodology for estimating relative task effort. Based on graph comprehension research and our motivational eye tracking experiments, we hypothesize a set of factors that should be taken into account in a model of task effort. We present our model, implemented in the ACT-R framework, and discuss the results of a final set of eye tracking experiments that validate our model as a predictor of relative task effort.