Not so simple. It's not going to be direct lines of transit. The Mass Relays aren't laid out for the convenience of the Krogan, or for the convenience of Humanity, etc etc. They are scattered among star systems, and only link to their pairs, or to nearby relays, depending on what kind of relay they are.
There's also the matter of many different factions controlling different assets within the same cluster. It would be simpler just to mark locations with color coding or some such, as opposed to rendering a spiderweb from hell across the galaxy.