Gridlines: What They Mean on a Chart
TERMFaint reference lines that help you read values off a chart.
Gridlines are the faint horizontal and vertical lines that run across a chart's plot area from the axis tick marks. They are not data. They are reading guides that let your eye travel in a straight line from any bar, point or line back to the axis, so you can estimate its value accurately instead of guessing.
So when you see light horizontal lines behind a graph, they almost always mean "this level equals the value labelled on the axis". The line at 60 marks every position where the value is 60. Vertical gridlines do the same job for the horizontal axis, usually marking dates or categories.
How to read a graph with gridlines
- Read the axis labels first. Note what each gridline stands for and the interval between them — in the chart above, every gridline is 20 units apart.
- Find your data point. Locate the top of the bar, the dot, or the spot on the line you care about.
- Find the nearest gridlines. Identify the gridline just below and just above the point.
- Interpolate. Estimate how far between the two the point sits. Halfway between 60 and 80 is 70; a quarter of the way is 65.
- Check the scale type. On a linear scale halfway really means the average of the two gridlines. On a log scale it does not: halfway between the 10 and 100 gridlines is about 32, not 55.
Treat the result as an estimate. If the exact figure matters, look for data labels or the source table.
Types of gridlines
| Type | What it does | Typical use |
|---|---|---|
| Horizontal (major) | Extends each labelled tick of the vertical (y) axis across the plot | Reading values on bar, column and line charts |
| Vertical (major) | Extends each tick of the horizontal (x) axis upward | Marking dates on time series; scatter plots |
| Minor gridlines | Fainter lines between the major ones | Charts that need finer reading, such as engineering or science plots |
| Reference line | A single emphasised line at a meaningful value (target, average, threshold) | "Target = 75%", "Average", a zero line on profit/loss |
Gridlines are generated from the tick marks of an axis, so they always share its scale and interval. Most bar and line charts need only horizontal gridlines, because the value you read is on the vertical axis. A horizontal bar chart flips this and uses vertical gridlines.
When to use gridlines — and when to drop them
Gridlines help when readers need to estimate values, compare points that are far apart, or read a point in the middle of a wide plot that is a long way from the axis. They add little when every bar already carries a data label, when the chart is a quick trend sketch where the shape is the message, or on a pie chart, which has no axes at all.
Make gridlines the quietest thing on the chart: thin, light grey (or dark grey on a dark background), solid or dotted, and behind the data. If you notice the gridlines before the bars, they are too heavy.
Common gridline mistakes
- Too many lines. Gridlines every 5 units on a 0–500 axis turn the chart into graph paper. Four to six major gridlines is usually plenty.
- Heavy or coloured lines. Dark, thick gridlines compete with the data — a classic form of chartjunk.
- Odd intervals. Gridlines at 0, 17, 34, 51 are hard to read. Use round steps such as 1, 2, 5, 10, 20, 25, 50.
- Gridlines in front of the data. Lines drawn over bars cut them into pieces; keep them on the bottom layer.
- Confusing gridlines with a trend or threshold. A plain gridline marks a value on the scale; it carries no meaning about the data. A target or threshold line should be styled and labelled differently.
Gridlines on maps and in design
The same idea appears elsewhere. On maps, the grid of latitude and longitude lines (or a lettered/numbered grid) lets you locate a place by coordinates, just as chart gridlines let you locate a value. In graphic and web design, a "grid" is an invisible set of columns and rows used to align content; it is a layout tool rather than a reading aid, and is usually not visible in the finished design.
Frequently asked questions
What are gridlines in a chart?
Gridlines are the faint horizontal and vertical lines that run across a chart's plot area from the axis tick marks. They are not data; they help you line up a bar, point or line with its value on the axis.
How do you read a graph with grid lines?
Read the axis labels to learn the interval between gridlines, find the data point, identify the gridlines just above and below it, and estimate how far between them it sits. Halfway between the 60 and 80 lines is about 70.
What do the horizontal lines on a graph mean?
Horizontal lines across a graph are usually gridlines. Each one marks a value on the vertical axis, so every point touching the 60 line has a value of 60. A single labelled line may instead be a target, average or threshold line.
What is the difference between gridlines and tick marks?
Tick marks are short marks on the axis itself at each labelled value. Gridlines extend those same tick positions across the whole plot area so you can read values far from the axis.
Should every chart have gridlines?
No. Use light gridlines when readers need to estimate values. Drop them when every bar has a data label, when the overall shape is the only message, or on charts without axes such as pie charts.