Chart gridlines are great for making the chart data more readable and detailed, Excel allows you to add major and minor gridlines to a chart. The major gridlines coincide with axis values and major tick marks.

How to insert

1. Select the chart.
2. Press with left mouse button on "plus" sign.
3. Press with left mouse button on checkbox "Gridlines".
   
4. Press with left mouse button on the arrow to expand options.
   
5. Now select the gridlines you want on your chart:
   • Primary Major horizontal
   • Primary Major vertical
   • Primary Minor horizontal
   • Primary Minor vertical
   • Secondary Major horizontal
   • Secondary Major vertical
   • Secondary Minor horizontal
   • Secondary Minor vertical

The following chart shows both major and minor gridlines in an x y scatter chart.

How to change gridline interval

1. Double-press with left mouse button on with left mouse button on axis values to open the task pane on the right side of your Excel window.
   Double press with left mouse button on the x axis values if you want to change the interval of vertical major and minor gridlines and vice versa.
2. Go to tab "Axis Options" on the task pane.
3. Press with mouse on "Axis Options" arrow to expand settings.
4. Change the major and minor units in order to change the gridline interval.