Interpret a graphic

[DaveO]

I've been reading the Miller TIG manual, and have a question about a graphic. This clip from the manual discusses the AC cycle, and describes the change from electrode positive to electrode negative that occurs while welding.

All well and good, but the graphic labeled Figure 2.14 shows a horizontal arrow pointing right in the positive half of the cycle, and a horizontal arrow pointing left in the negative half. That pointing-left arrow is bugging me: it seems to contradict the implied time continuum of the X axis.

What do those arrows indicate?