Abstract: Axis grid is a basic reference structure that assists people in reading line charts.In this paper,we focus on axis labels that help people perceive slopes in line charts.A series of user studies are conducted to explore the influence of local aspect ratio on the task of slope estimation in line charts.We evaluate the influence of different experimental conditions by calculating the errors between the user estimated slope and the ground truth.The re-sults show that appropriate grid density and squarified grid aspect ratio increase the estimation accuracy of line segment slopes.Accordingly,we propose a new algorithm for generating axis grid considering the maximal label,the minimal label and label intervals for line charts.Following the previous criteria of simplicity,coverage and overlap,our algorithm applies the squarifed local aspect ratio to the joint optimization of the tick marks of both horizontal and vertical axis to generate aesthetic axis grid that helps increase graphical perception accuracy.