UpDown Meter can graph network activity for a selected network adapter.
On the first run, you are asked to select your network card with a question mark this isn't clickable. Instead, you want to go to options and pick your network card.
The interface is minimal, if not a bit small. That said, you can easily see red spikes as well as your download and upload raw and averages. Small buttons allow you to set the transparency, always on top, and clear the graph. A few options are also available including the adapter selection mentioned above including disabled adapters and settings for the reported and actual speeds. One more tab here lets you choose to start with Windows, load minimized, screen docking, and helpful tips.
An installer version is available at the author's website. We're proving the portable version because that's just the best way to do it.
If you need to monitor your bandwidth, or look for problems, UpDown Meter is unobtrusive, uses little resources and is simple enough for anyone to figure out.