Data Status is a traffic monitoring app that gives you live updates via the notification dock. It’s free for the first month and then it costs $1 dollar for a lifetime license.
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Pro Notification stats
Shows an icon and the usage data at the system notification.
Pro Homescreen widget
Has a homescreen widget shows the pie/donut-style graph of how much data and how many days are left in the current cycle.
Con Does not have breakdown by app
The app can show overall usage statistics, but it can't show how much data was used by each app.
Con No recent updates
Last update was in June 2015.