This app was mentioned in 3 comments, with an average of 2.00 upvotes
Unfortunately it's not a widget yet, but for a standalone SNMP grapher, this is OK: https://play.google.com/store/apps/details?id=snmptrafficgrapher.app
Only issue I have is with some routers, it loses the connection to the PPP entry if the connection drops.
5 minutes later, I've got one too!
Seeing that this is just SNMP, this app lets you change the OIDs, maybe I can graph other data (current TCP connections, ping to Google, RAM/CPU on my ESXi server).
For everyone interested, I'm using the same app as OP, SNMP Manager, It autopopulates the router IP, just need to scan interfaces and pick the one you want to watch.
No. I have pfSense installed on a HP Microserver. I use this app installed on the phone to generate the graphs.