Horizontal axis represents time intervals in hours. Corresponding graph shows data usage during the day. To consult mobile data usage during particular interval of time login to the router via SSH or go to the Services -> CLI page and execute the following command: mdcollectdctl [OPTIONS] . OPTIONS field varies from your needs. More information on mdcollectdctl command and its options can be found here.
Data traffic statistics are stored in /tmp/mdcollectd.db file. If you are using a Linux-based OS go to the directory on your PC where you want to relocate the files, right click anywhere and choose the "Open in Terminal" option. In the Terminal command line use the Secure Copy (scp) command to copy the files from the router. The full command should look something like this: $ scp email@example.com:/tmp/mdcollectd.db ./
If you are using Windows, you can copy the files using WinSCP freeware, more info on that here