Formatting the Gridlines

The gridlines of a chart can be customized (pie, indicator, heat map and organization charts do not have gridlines). To do this:

Right-click any chart element and select the desired entry from the Format Gridlines submenu to bring up the corresponding Format Gridline dialog.
Specify the line color, style, transparency, and thickness of the gridline (to change the color, click the color image and select a color from the color palette or input the hexadecimal value of a color directly in the text box).
When done, click OK to accept the settings and close the dialog.

See also the following documents for detailed explanation about options in each dialog: