Any good application for data usage monitor?

I was in a similar situation, but with slow speeds after a certain amount of data usage is reached (now I have a 16 GB 12 Mbit plan ☺) and I used vnstat (package vnstat). It's a command-line app that displays the bandwidth usage for today and previous days. as soon as it is installed, it will start monitoring the usage.

Usage: run vnstat to view the statistics, vnstat -d for viewing daily usage. Add -i wlan0 to the previous command if you are using a wireless network. See the manual page for it by running man vnstat for more information.

vnStat - Light Weight Console-based Network Monitor

vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network traffic for the selected interface(s). It uses the network interface statistics provided by the kernel as information source. This means that vnStat won't actually be sniffing any traffic and also ensures light use of system resources.

In this tutorial we'll review:

Features
Installation
Configuration
Start Systemd Service
Usage (from command line)
Conky Real Time Display example

Features

quick and simple to install and get running
gathered statistics persists through system reboots
can monitor multiple interfaces at the same time
several output options
summary, hourly, daily, monthly, weekly, top 10 days
optional png image output (using libgd)
months can be configured to follow billing period
light, minimal resource usage
same low cpu usage regardless of traffic
can be used without root permissions
online color configuration editor

Installation

nvStat is in the official repositories so no need to link to a new ppa. To install create a Terminal instance using Ctrl+Alt+T and type at the prompt:

sudo apt-get update
sudo apt-get install vnstat

After installation, keep your Terminal open for the following sections. There is no need to reboot.

Configuration

Pick a preferred network interface and edit the Interface variable in the /etc/vnstat.conf accordingly. To the list all interfaces available to vnstat, use:

$ vnstat --iflist
Available interfaces: wlp60s0 lo enp59s0 (1000 Mbit)

To start monitoring a particular interface you must initialize a database first. Each interface needs its own database. The command to initialize one for the eth0 interface is:

sudo vnstat -u -i enp59s0

Start Systemd Service

After introducing the interface(s) and checking the config file. You can start the monitoring process via systemd:

sudo systemctl start vnstat.service

To make this service permanent use:

sudo systemctl enable vnstat.service

From now on vnstat will be gathering network usage in the background using such a small percentage of CPU it doesn't show up on conky's (system monitor's) top 9 list of processes (on my machine).

Usage (from Command Line)

Query the network traffic:

vnstat -q

Viewing live network traffic usage:

vnstat -l

To find more options, use:

vnstat --help

Monthly Totals

To see monthly totals, use:

$ vnstat -m

 enp59s0  /  monthly

       month        rx      |     tx      |    total    |   avg. rate
    ------------------------+-------------+-------------+---------------
      Oct '17      2.02 GiB |    1.57 GiB |    3.59 GiB |   11.25 kbit/s
      Nov '17     58.28 GiB |   24.58 GiB |   82.86 GiB |  268.17 kbit/s
      Dec '17    143.23 GiB |   13.64 GiB |  156.87 GiB |  491.31 kbit/s
      Jan '18    102.77 GiB |   30.21 GiB |  132.97 GiB |    1.04 Mbit/s
    ------------------------+-------------+-------------+---------------
    estimated    257.06 GiB |   75.56 GiB |  332.62 GiB |

Conky Real Time Display example

Conky is a popular light-weight System Monitor used across many Linux distributions. You can show vnStat bandwidth totals in your conky display like this:

Conky Network Realtime 5.gif

30 second .gif of Conky's relevant section

The Conky code to produce this display is:

${color orange}${voffset 2}${hr 1}
${color}Memory:${goto 148}${color green}$mem / $memmax $alignr${color green}${memperc /}%
${color}Linux:${goto 148}${color green}${fs_used /} / ${fs_size /} $alignr${color green}${fs_used_perc /}%
${color}NVMe Win 10:${goto 148}${if_mounted /mnt/c}${color green} ${fs_used /mnt/c} / ${fs_size /mnt/c} $alignr${color green}${fs_used_perc /mnt/c}%${else}${color yellow}/mnt/c${endif}
${color}${if_mounted /mnt/d}HGST_Win10:${goto 148}${color green} ${fs_used /mnt/d} / ${fs_size /mnt/d} $alignr${color green}${fs_used_perc /mnt/d}%${else}Cache RAM:${goto 148}${color green}${cached} ${color} Buffers: ${color green} ${buffers}${endif}
${color}${if_mounted /mnt/e}WSL+Linux:${goto 148}${color green}${fs_used /mnt/e} / ${fs_size /mnt/e} $alignr${color green}${fs_used_perc /mnt/e}%${else}Swap:${goto 148}${color green}${swap} / ${swapmax} $alignr${color green}${swapperc}%${endif}
${color orange}${voffset 2}${hr 1}
${color1}Network using vnStat "-i", "-w" and "-m"
${color}${goto 5}Today ${goto 100}Yesterday ${goto 225}Week ${goto 325}Month ${color green}
${execi 10 vnstat -i enp59s0 | grep "today" | awk '{print $8" "substr ($9, 1, 1)}'} ${goto 110}${execi 10 vnstat -i enp59s0 | grep "yesterday" | awk '{print $8" "substr ($9, 1, 1)}'} ${goto 220}${execi 10 vnstat -i enp59s0 -w | grep "current week" | awk '{print $9" "substr ($10, 1, 1)}'} ${goto 315}${execi 10 vnstat -i enp59s0 -m | grep "`date +"%b '%y"`" | awk '{print $9" "substr ($10, 1, 1)}'}
${color}Down: ${color green}${downspeed enp59s0}/s ${color}${goto 220}Up: ${color green}${upspeed enp59s0}/s
${downspeedgraph enp59s0 25,190 000000 ff0000} ${alignr}${upspeedgraph enp59s0 25,190 000000 00ff00}$color
Total: ${color green}${totaldown enp59s0} $color${alignr}Total: ${color green}${totalup enp59s0}
${color orange}${voffset 2}${hr 1}
${color}${goto 5}Dawn: ${color green}${execpi 300 cat /usr/local/bin/sunrise} ${goto 155}${color}Dusk: ${color green}${execpi 300 cat /usr/local/bin/sunset} ${alignr}${color}Level: ${color green}${execpi 10 cat /sys/class/backlight/intel_backlight/brightness}
${color orange}${voffset 2}${hr 1}

To save desktop space, my narrow Conky window uses "G" instead of "GiB", "M" instead of "MiB", etc. If you have more screen real estate change substr ($10, 1, 1) to $10 and do the same for $9.

You may have to change enp59s0 to eth0, wlan0 or eth1, etc. depending on your network name reported by ifconfig.

You can check NTM - Network Traffic Monitor

There is Datafox, an addon for Firefox.

This should monitor at least your bandwidth usage on the web, I don't know if it can monitor usage with other applications however.

I needed protocol granularity to see what was using data on my network, so I opted for ntop. It has a web interface, and worked out the box for me.