Grid lines are lines that cross the chart plot to show axis divisions. Grid lines help viewers of the chart see what value is represented by an unlabeled data point. Especially for large or complicated charts, grid lines give valuable cues to the viewer.
Grid lines come in two types: major and minor.
Major grid lines separate the axis into major units. On category axes, major grid lines are the only grid lines available (you cannot show minor grid lines on a category axis.) On value axes, major grid lines are drawn for every major axis division.
Minor grid lines separate the units delineated by major grid lines. Minor grid lines, which can only appear on value axes, appear for every minor axis division.
By default, major grid lines appear for value axes. You can show or hide grid lines for both value and category axes.
The number of grid lines that display depends on the Major Divisions and Minor Divisions settings in the Scale tab of the Format Axis dialog.
To show or hide major or minor grid lines:
Note Since category axes have no minor divisions, they cannot display minor grid lines. Checking the Minor Grid checkbox has no effect on category axes.
By default, major and minor grid lines are black, so you can't tell the major grid lines from the minor ones. You can change the grid lines' colors and line styles to differentiate them from each other or to make them less conspicuous on your chart.
While major and minor grid lines may be different colors, all of the major grid lines on a chart must be the same color. All of the minor grid lines on a chart must likewise be the same color.
For information on how to change the grid line colors and line styles, see Changing Line Styles and Colors.
