Use gridlines sparingly. There should usually be between four and eight gridlines per chart, depending on the size of the chart and the level of data. They should provide enough reference points to read the data values in the chart easily.
Number of gridlines
Do not use too many or too few gridlines.
Example of too many gridlines
Example of too few gridlines
Example of correct number of gridlines
Choose sensible and easy to read and follow gridline intervals.
Example of good gridline intervals
Example of difficult to follow gridline intervals
Gridlines should always be grey: RGB value (190,190,190).