1) Improve efficiency for large sheets

2) Complete graphics (pie, histogram)
  -  Graphs cannot be resized properly !!  Need to fix this.
  -  A graph configurator so that you can change colors, axis limits,
      etc...
  -  Changing the data in the spreadsheet should update the graph
      automagically.
  -  Fix the code to be more reliable against wrong use of
      the graph fonctionality
  -  Add documentation

