I tried with mine and it doesn’t give me the floating graph either. I tested with three different browsers. So im not sure why the floating graph is not working with Weathermaps.
I did find this tho.
I’ve defined some OVERLIBGRAPH lines, but nothing appears in the map HTML when I use the command-line tool.
For historical reasons, the default format for HTMLSTYLE is ‘static’ which avoids javascript. To get the overlib graphs to appear you need to have ‘HTMLSTYLE overlib’ near the top of your map configuration file.
from http://network-weathermap.com/manual/0.97b/pages/faq.html