The time arrow goes from left to right. The up-and-down lines rather cleverly show the proximity of the main people at different times. For instance, Judas is the black line. He is with the others at the beginning of the Last Supper, then breaks away, only to come back at the arrest of Jesus.
If zooming in doesn't work well enough, you can click and go to the original, which is bigger, and higher resolution.
Strict chronology was not the purpose of the Gospels, so any harmonization is doomed. Even so, the Bear found this interesting. (He was wondering what happened between Palm Sunday and Holy Thursday.)