Gridlines

Gridlines are the faint lines running across a chart's plot area from the axis tick marks, there to help the eye line up a data point with its value.

Tick marks alone sit only at the edge of the chart, so judging whether a bar in the middle of a wide plot reaches 60 or 70 can be hard. Gridlines extend those ticks inward as light guides, letting you trace a straight path from any point back to the axis. They are deliberately subtle — usually thin and low-contrast — because they are scaffolding for reading, not data themselves.

How gridlines work

Gridlines mirror an axis's scale: a horizontal gridline is drawn at each major tick of the vertical axis, and a vertical gridline at each tick of the horizontal axis. Most charts show only horizontal gridlines, since the value being read is usually vertical. Major gridlines mark the labelled intervals; some charts add fainter minor gridlines between them for finer reading. Used sparingly they aid accuracy; overused, they clutter the plot and compete with the data.

A concrete example

In a line chart tracking a metric over twelve months, horizontal gridlines might sit at 0, 20, 40, 60, 80 and 100. To read the value at a particular month, you find the point on the line and see which gridlines it falls between — say just above the 60 line — giving you a quick, reliable estimate without measuring against the distant axis. Remove the gridlines and that same estimate becomes guesswork.

Related terms

Gridlines are generated from an axis and its scale, and they exist to help you read the value of each data point. You will see them at work in nearly every plotted chart, including the examples in the line chart guide.