Lines drawn in the chart plot area to ‘aid in the visual alignment of data’ are called grid lines. Charts can have either vertical grid lines or horizontal grid lines or both.
Fig: Column chart with horizontal grid lines
Fig: Bar chart with vertical grid lines
Fig: Bubble chart with horizontal and vertical grid lines
Many people consider grid lines as an useless carry over from an age when charts were drawn by hand on grid paper and therefore needed additional lines as reference points to plot data accurately. They think that in today’s era of software generated charts, grid lines have become redundant.
However, there are certain scenarios in which grid lines can be useful:
Scenario 1: To add precision in large graphs
For graphs, where data points are too far from the quantitative axis, it helps when you have grid lines to align the data to the right value.
It would have been difficult to the measure of the value for December 2013 from this chart had the grid lines not been there.
Scenario 2: To bring out subtle differences
Can you detect the difference between the monthly revenues of March and April from the chart below?
Due to their almost similar lengths, it is difficult to find out their difference.
If we redraw the same chart using grid lines, it would be easier to detect their difference.
Scenario 3: To focus on a specific area in bubble or scatter chart
In a bubble or scatter chart, it is difficult to focus on a specific area because data values are scattered.
The grid lines, in this case, divide the graph into sections and help users to focus on a specific section for analysis.
To know about other scenarios where grid lines can be useful, we recommend reading Stephen Few’s article on Grid Lines in Graphs are Rarely Useful.
How to use grid lines effectively?
Now that we know that grid lines can help in interpreting data in certain scenarios, here are three tips to make them more effective:
Keep them subtle
Visually heavy grid lines distract the user from the actual data.
If you are using grid lines in your chart, make sure they are subtle and do not hinder with the interpretation of data.
When adding trendline with grid lines
A trendline is used to add context to a chart, by helping the user interpret data with respect to some predetermined value. It provides a reference point to compare the measure against a set benchmark.
In the above chart, the Target line is a trendline. When adding a trendline along with grid lines, ensure that the trendline is of a different color and visually stands out from the other grid lines.
[Related read: How to add trendlines in FusionCharts Suite XT?]
When adding regression line with grid lines
At times, you may be required to plot a regression line to get a sense of the trend from a scatter plot.
Similar to adding a trendline, ensure that the regression line is of a different color to that of the grid lines. Ideally, the regression line should match the color of the respective data plot to ease the process of interpretation for the user.
Now over to you. Do you think grid lines are chart junk or visual aids? Share your thoughts in the comment section below.