This is becoming a FAQ. (Alternately, add a config setting that would allow period=year.)
We also have to update the FAQ on how unique is counted. (It refers to third party cookies).
I updated FAQ: http://piwik.org/faq/general/#faq_43
but no config setting, since at some point we will not keep more than monthly logs so might as well not build the feature that will be broken ;)
actually vipsoft is right, we need a setting!
(In ) Fixes #2170 New setting to enable Unique Visitors for Year/Custom Range
; if set to 1, Unique Visitors will be processed for Years and Date Ranges
; disabled by default, to ensure optimal performance for high traffic Piwik instances
; if you set it to 1 and want the Unique Visitors to be re-processed in for reports in the past, drop all piwikarchive* tables
enable_processing_unique_visitors_year_and_range = 0