Bank to ~45° for Slope Perception.

The eye reads change in a line chart by judging slope. When most segments hover near 0° (too wide) or 90° (too tall), trends become hard to compare. Cleveland's banking rule says to choose an aspect ratio so the absolute slopes average around 45° - the angle at which differences in slope are easiest to discriminate.

William Cleveland, 1988 · The Shape Parameter of a Two-Variable Graph