Bug in showing unique visitors in the tab Visitor and the report Overview?

When I go to the tab Visitors and look at the report Overview I can see my unique visitors below the graph in numbers (as shown in picture 1 and the green frame).
But when I change the date to a range or the entire year I can not see any unique visitors below the graph. Picture 2 shows the unique visitors missing at the red line. I would like to be able to see my unique visitors when I choose the whole year or a date range. The unique visitors show when I look at a day or month. Is this how Piwik is suppose to work or is this something you plan to fix?
All my best regards,