Vnstat for internet data metering

To keep track of your internet data consumption, I recommend vnstat.  To install it, log onto the internet, open a terminal-window (CTRL-ALT-T) and enter "sudo apt-get install vnstat." Then, to check your data usage, all you have to do is open a terminal-window and enter "vnstat." At least that's all it took in my case.  Vnstat can do a lot more, too, but it's so compact, and so easy to install and use, that there's no point in searching for something dedicated to monitoring internet data consumption, or something with a graphical interface.