The 'vertical' axis is spatial, representing position across the track, but I would argue that if we take your final sentence and append "...at sequential time

The 'vertical' axis is spatial, representing position across the track, but I would argue that if we take your final sentence and append "...at sequential time intervals" to it, we end up making the case that the horizontal axis in the image is actually time, not space. Each column is the finish line at a different time, and perhaps counter-intuitively (given the usual convention), the right-most column is the earliest. This is clearer, I think, from considering how the banner is made: as others have pointed out, it is produce by having a single row of LEDs in line with the finish line, each one being turned on and off in a sequence which results in the banner shown in the picture.