The problem:
When I draw a "Combination chart: lines and columns" the line in this chart has 
icons (small symbols on each point) on it (default). When you first remove these 
icons and later on you put them back on the chart they will appear on the screen, 
but you can't save them anymore. When you re-open the file they are gone.

Steps to repeat the problem:
Draw a "combination chart: lines and columns"
Remove the icons on the line
Save and close the file
Reopen the file (no icons on the line)
Choose new icons on the line
Save the file
Close and reopen the file
Icons are gone