Your link has no TARGET. That's what weathermap uses to get data. The infourl and overlibgraph are just to pop that graph on the screen, drawn by Cacti. Weathermap can't read PNG files

So if you go to the Cacti graph page for that graph and click the little wrench icon, you'll see the rrdtool command that was used to draw that graph. In there is the full filename for the rrd file. Use that in the TARGET line for the link. You will probably also need to change the DS names, since this isn't a normal traffic graph. You'll be able to see those in the rrdtool command too, but if you just add:
TARGET /var/blah/whatever/file.rrd
as a target, Weathermap will complain that there is no valid data, and tell you the possible DS names for that rrd file in the logs.
Then you can:
TARGET /var/blah/whatever/file.rrd:in_ds:out_ds
with the correct names to represent data flowing in and out of the first node (in the NODES line).