Steps to reproduce:
Open Visitors Overview:
Add metrics to plot: "Unique Visitors"
-> Everything looks fine.
Now select period = year:
Add metrics to plot "Unique Visitors"
-> Displays 0 for year 2011 and 2012.
Same for project "virtual drums", "Piwik Mobile", ...
Other metrics are displaying a value for year 2011 and 2012.
Don't know whether it's a bug or whether it's just missing data in our database...
There is a config parameter "enable_processing_unique_visitors_year_and_range = 0" which is disabled by default, because it is very performance intensive to process this metric. I could imagine, this is also disabled in the demo instance of piwik.