Click on the 'metric data' tab, click on 'show all metrics' and then set a collection interval on:
Network Usage (Average)
And click the play button on the bottom to enable collection of that metric. Since its not a default indicator metric, you'll have to click on the drop down that says 'problem metrics' and change it to 'all metrics', this will change the lower portion of the page to show all the metrics that are being collected for the resource you are looking at. Click the play button to add the metric to the graphs.
