In simple graphs that only have arrows at the axes I leave out these arrows and put in a zero or origin sign. If a key is needed, put it where it will be read ahead of the drawing proper. If it has to be in another place, notify the reader of this in your title. The columns of a key shouldn't be too far apart. On a page that's to be rotated, the key should preferably be written in the drawing's direction.
Some drawings show a sequence of events, a progression or a flow of things.