This example is the second of three in the Path Transitions tutorial; see the previous example for context.
The desired pairing of numbers for path interpolation is like this:
M x0, y0 L x1, y1 L x2, y2 L x3, y3 L xR, y4
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
M xl, y0 L x0, y1 L x1, y2 L x2, y3 L x3, y4
Where xl is some negative value off the left side, and xr is some positive value off the right side. This way, the first point ⟨x0,y0⟩ is interpolated to ⟨xl,y0⟩; meaning, the x-coordinate is interpolated rather than the y-coordinate, and so the path appears to slide off to the left. Likewise, the incoming point ⟨xr,y4⟩ is interpolated to ⟨x3,y4⟩.