NPM focuses on specific devices. If you want a graph that shows your bandwidth for your entire network you will need to create a Multi Series Cart and add all of your network interfaces to it.
Alternatively, you could create a summary view with a separate interface chart for each of your critical network devices/interfaces that you wish to track.
Ultimately how you set this up will be dictated by your specific use case; as an example we track the bandwidth usage at our border so I have a summary page setup showing the border interfaces as well as Multi Series Charts showing the combined usage.
Hope this helps!