Change bubble shapes in bubble charts

To change the shape of all the bubbles in a bubble chart:

In the External Style Sheet editor, under Data series, select Bubble series, or in the Inline Style Sheet editor, under Data series, select Series.
On the right-hand area of the editor you are using, select the data series entry from the Series list, click the More options button at the bottom right-hand corner of the right side of the area, and then find the Bubble type setting group in the dialog that turns up (see the picture below).
Select the shape you want the bubbles to look like from the available options under the group.