Gridlines Mitigate Sine Illusion in Line Charts

TL;DR

This study investigates how gridlines can reduce the sine illusion in line charts, finding aligned gridlines most effective and developing a model to predict illusion impact based on line ratios.

Contribution

It introduces a model predicting sine illusion effects in line charts and demonstrates that aligned gridlines significantly mitigate the illusion.

Findings

Aligned gridlines most effectively reduce the sine illusion.

Participants are influenced by the illusion when the ratio is below 50%.

The equal triangle explanation better predicts participant behavior.

Abstract

Sine illusion happens when the more quickly changing pairs of lines lead to bigger underestimates of the delta between them. We evaluate three visual manipulations on mitigating sine illusions: dotted lines, aligned gridlines, and offset gridlines via a user study. We asked participants to compare the deltas between two lines at two time points and found aligned gridlines to be the most effective in mitigating sine illusions. Using data from the user study, we produced a model that predicts the impact of the sine illusion in line charts by accounting for the ratio of the vertical distance between the two points of comparison. When the ratio is less than 50\%, participants begin to be influenced by the sine illusion. This effect can be significantly exacerbated when the difference between the two deltas falls under 30\%. We compared two explanations for the sine illusion based on our data: either participants were mistakenly using the perpendicular distance between the two lines to make their comparison (the perpendicular explanation), or they incorrectly relied on the length of the line segment perpendicular to the angle bisector of the bottom and top lines (the equal triangle explanation). We found the equal triangle explanation to be the more predictive model explaining participant behaviors.