The chart webparts for Tasks in % Completed display bad value for other the US regionals settings.
The decimal separator in EU is the comma not the dot. So when for EU site the %Compleded show 20% if fact the value in the db is 0,20. The web parts take this value for 200% and show a full 100% value.
Calling Visifire forum , no solution exist at this time.
The workaround is to define a new text calculated field with =REPLACE([% Complete];SEARCH(",";[% Complete]);1;".") and select this new field as data.