support/graph-size: report 'Unknown" after all packages, but before "Others"

support/graph-size: report 'Unknown" after all packages, but before "Others"

Currently, the "unknown" category may be reported anywhere, so it does
not really stand out when there are a lot of packages in the graph.

Move it towards the end, but right before the "other" category, so that
it is a bit more visible. Like for Others, don't report it if its size
is zero.

Also, make it title case (i.e. "Unknown" instead of "unknown").

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>