The Fill Patterns dialog box allows you to create different types of patterns in your chart.
To open the Fill Patterns dialog box, type Alt+O and F while in the Chart window.  Use the 
mouse to click on the portion of the graph where you want to change the pattern, then choose
the pattern you want by clicking on the pattern sample on the left side of the dialog box.
When finished, type the Tab key until the OK button is highlighted, then type Enter.