Somewhat confusingly, the vertical axis is the horizontal displacement against time on the horizontal axis.
This graph is from Lissmann (1945). It shows the horizontal displacement in time of a mucus gland on the sole of a Helix pomatia, a land snail, that was crawling up on a vertical glass plate.
The most significant piece of information to be gained from this graph is that the movement of a point, any point, on the sole of a snail is not continuous, but intermittent. That is the case even though the overall movement of the snail is continuous.
H. W. LISSMANN. 1945. The Mechanism of Locomotion in Gastropod Molluscs: I. Kinematics
J. Exp. Biol. 21:58-69. pdf