Hi Ahmed ,
You can use a ListBox with a change in the ListStyle property to fmListStyleOption ; this will display a checkbox against each item in the ListBox.
The more important point is that checkboxes should be visible on the chart for two reasons , so that the viewer knows :
1. what options have been selected
2. what are all the options available
If you use a drop-down list , neither of the above will be possible , assuming that multiple selections are possible.
If arranging the checkboxes horizontally , at the bottom of the chart , is not neat , arrange them vertically , to the right of the chart , so that the checkbox text is to the right , and more checkboxes can be accommodated in the available space.
Narayan