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
Select the chart.
Click "plus" sign.
Click checkbox "Gridlines".
Click the arrow to expand options.
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
Double-click with left mouse button on axis values to open the task pane on the right side of your Excel window.
Double click the x axis values if you want to change the interval of vertical major and minor gridlines and vice versa.
Go to tab "Axis Options" on the task pane.
Click on "Axis Options" arrow to expand settings.
Change the major and minor units in order to change the gridline interval.