you can see historical network IO (or any other statistics) when you create a "Top-N: widget in a Dashboard, insert SQL query and select "Net Throughput" as an Overlay graph.
If you want to sum the Network Throughput of all VMs you can do that via Trends:
1. perform an SQL query ( i.e. "vm.folder.name:Folder1") and press "Trend Results" button
2. keep Scope tab unchanged, on "Criteria" tab choose an attribute you want to see (e.g. "vm.netThroughput") and save the Trend
3. In Reporting - Business Views you cans see the new trend (it's necessary to switch from Facets to Trends view on the top left corner).
4. In any Dashboard you can create a new "Trend" widget and display the newly created trend (or more trends in one widget), eventually export the results into the PDF format.
Note: by default the Trends job is launched every 4 hours so the Attribute should return "hour" statistics, not "latest". Period of the Trend job can be modified in Setup - Advanced Setup - System Properties menu, "Hourly interval for Trend Execution" property. But it's not recommended that the value is changed.
Let us know if you have additional questions or comments.