A method is performed at an electronic device. The process displays a line chart, which includes a first plurality of data marks and a first plurality of line segments connecting adjacent data marks. The process detects a touch input, and expands the line chart horizontally, including expanding a first line segment. The process adds a second plurality of data marks on the first line segment, thereby subdividing the first line segment into a second plurality of line segments, which are initially collinear. After expanding the chart and adding the second plurality of marks, the process determines an ordinate value for each of the second plurality of marks, and animatedly moves each of the second plurality of marks from the first line segment to a respective vertical location defined by the respective determined ordinate value. The process concurrently moves each of the second plurality of line segments according to its endpoints.