Bug#678819: vnstati: Output to stdout does not work

[nodiscc <nodiscc@gmail.com>]

Package: vnstati
Version: 1.11-1
Severity: normal

Dear Maintainer,
The manpage for vnstati says:
	-o, --output file
              Write png image to file and exit. Output can be directed to std‐
              out by giving "-" as filename.

However, it looks like output to stdout is not supported

$ vnstati -i wlan0 -o -
At least output mode and file parameter needs to be given.
Use -? or --help for getting short help.

I've tried piping it to an image viewer, unsuccessfully. Is the manpage outdated?
This feature would be welcome :)

Thanks

-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-2-amd64 (SMP w/4 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages vnstati depends on:
ii  libc6       2.13-33
ii  libgd2-xpm  2.0.36~rc1~dfsg-6.1
ii  vnstat      1.11-1

vnstati recommends no packages.

vnstati suggests no packages.

-- no debconf information