Or "why SVG, it doesn't tell me anything". Not true, there are many uses for real time graphing data that MRTG, ifgraph and similar historical packages cannot provide. These fill two different needs.
Not directly on the firewall. These packages all have heavy requirements like Perl and others. In order to keep m0n0wall light, these packages cannot be added directly to the system. m0n0wall's file system design, in that it runs from RAM and does not maintain anything other than your configuration across reboots, is not condusive to applications of this nature.
You can run these from another system on your network. See ifgraph section of this guide.