Right click on the data series you wish to change, choose “Format Data Series” and go under “Marker Fill” menu from the left. Choose the option “Gradient Fill” and from the selections that appear below first determine the type of colors you wish to appear on the markers under the “Preset colors” menu. Under “Type” determine whether the lining is linear, radial, rectangular or path. Under “Direction” determine in which way the type previously selected appears. For Linear lining you can also determine the angle under “Angle”. Under “Gradient Stops” area you can determine the colors your gradient setting will use. Click on “Close” if you are satisfied.