vnstat -m

 eth0  /  monthly

       month        rx      |     tx      |    total    |   avg. rate
    ------------------------+-------------+-------------+---------------
      Apr '15    371.81 MiB |  147.02 MiB |  518.83 MiB |    1.64 kbit/s
      May '15    136.39 MiB |   44.07 MiB |  180.46 MiB |    0.55 kbit/s
      Jun '15    139.16 MiB |   22.30 MiB |  161.46 MiB |    0.51 kbit/s
      Jul '15    289.27 MiB |   71.35 MiB |  360.62 MiB |    1.10 kbit/s
      Aug '15    126.22 MiB |   38.92 MiB |  165.15 MiB |    0.51 kbit/s
      Sep '15    109.10 MiB |   13.36 MiB |  122.45 MiB |    0.39 kbit/s
      Oct '15     92.87 MiB |   12.13 MiB |  105.00 MiB |    0.32 kbit/s
      Nov '15    123.39 MiB |   49.38 MiB |  172.76 MiB |    0.55 kbit/s
      Dec '15    184.05 MiB |   94.87 MiB |  278.92 MiB |    0.85 kbit/s
      Jan '16    180.21 MiB |   36.92 MiB |  217.14 MiB |    0.70 kbit/s
    ------------------------+-------------+-------------+---------------
    estimated       188 MiB |      37 MiB |     225 MiB |