TAChart: Add specific legend item for box-and-whiskers series

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@39932 4005530d-fff6-0310-9dd1-cebe43e6787f
TAChart: Update "multi" demo to show box-and-whispers legend

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@39933 4005530d-fff6-0310-9dd1-cebe43e6787f