Following webs or links are valuable (please read disclaimer):

German links             see there
Excel-Newsgroup (Google) All ever posted usenet Q+A concerning excel in general (english)
  same for: charting     ........................................... charting
  same for: programming  ........................................... VBA and Excel4Macros
                         and so on for other specific fields!
peltiertech.com          wonderful instructions for great charts not built in into excel 
Excel-4 macro functions  Microsoft documentation as a help file
Chart labeler            Attaches labels close to chart data series Excel does not provide

(frequent additions)