Chart Terminology: Chart Terms A–Z

GLOSSARY

The vocabulary of charts, defined in plain English.

Chart terminology is the vocabulary used to describe the parts of a chart and the data it shows. The core terms are the axis, scale, tick marks, gridlines, axis titles, legend, data series, data points and data labels. This glossary defines each of them, plus key statistics and chart-type terms, in one plain-English line.

Knowing these words makes any chart easier to read, describe and critique. Below you will find a labelled diagram of the parts of a chart, the full A–Z list of chart terms with one-line definitions (each links to a fuller page with examples), and answers to common questions about chart terminology.

Parts of a chart, labelled

A line chart with its parts labelled: chart title, y-axis, axis title, tick marks, gridlines, data series, data point, data label, legend and x-axis Monthly visitors ← chart title 3002001000 Visitors (thousands) 260 JanFebMarApr Month Site A Site B ↑ legend ← data label ← data point ← data series ← gridline ← x-axis ↑ x-axis title Rotated text on the left = y-axis title; 0–300 = tick labels
Most of the vocabulary of charts in one picture. Each term is defined in the A–Z list below.
PartWhat it tells you
Chart titleWhat the chart is about, often with the period and units
Axes (x and y)The two reference lines that give each mark its position
Scale, tick marks and labelsThe divisions on each axis and the value each one stands for
GridlinesReading guides that extend the ticks across the plot
Axis titlesWhat each axis measures and in which unit
LegendWhich colour or symbol belongs to which series
Data series and data pointsThe data itself
Data labelsExact values printed on the marks

Chart terms A–Z

A

Area chartA line chart with the space below the line filled in to emphasise volume over time. AxisA reference line along the edge of the plot that gives every mark a position and a value. Axis titleThe label naming what an axis measures, ideally with its unit, e.g. "Revenue ($ thousands)".

B

Bar chartA chart that compares categories using bars whose length is proportional to their value. BaselineThe reference value, usually zero, that bars and areas are measured from. BinOne of the equal-width value ranges a histogram groups numeric data into, such as 10–20. Box plotA chart showing a dataset's median, quartiles, range and outliers in one compact box and whiskers.

C

Categorical dataValues that fall into distinct named groups, such as country or product, rather than on a number line. ChartjunkDecoration that adds ink without adding information: heavy gridlines, 3D effects, needless icons. Continuous dataMeasured numbers that can take any value within a range, like height, time or temperature. CorrelationHow strongly two variables move together, measured from −1 (opposite) to +1 (together).

D

Data labelText printed on or next to a mark that states its exact value, such as "42%" on a pie slice. Data pointA single value plotted on a chart: one dot, one bar, one vertex of a line. Data seriesA related set of data points drawn together as one line, one colour of bars or one group. Donut chartA pie chart with the centre removed, often used to show parts of a whole with a total in the middle.

G

GridlinesFaint lines extending from the tick marks across the plot so values can be read accurately.

H

HeatmapA grid of cells in which colour intensity encodes each cell's value. HistogramA bar-like chart showing how many values fall into each bin of a continuous variable.

L

LegendThe key that explains what each colour, line style or symbol in a chart represents. Line chartA chart that connects data points with a line, mostly used to show change over time. Linear scaleA scale where equal distances always represent equal differences in value (0, 10, 20, 30). Log scaleA scale where equal distances represent equal ratios (1, 10, 100, 1,000), used for huge ranges.

M

MedianThe middle value when data is sorted; half the values are below it and half above.

O

OutlierA data point that lies unusually far from the rest of the data.

P

PercentileThe value below which a given percentage of the data falls; the 90th percentile beats 90% of values. Pie chartA circle divided into slices whose angles show each category's share of a whole.

Q

QuartileOne of three cut points (Q1, median, Q3) that split sorted data into four equal-sized groups.

R

RangeThe difference between the largest and smallest values in a dataset. Regression lineThe best-fit straight line through the points of a scatter plot, summarising their relationship.

S

ScaleThe rule that maps data values to positions along an axis, including its range and intervals. Scatter plotA chart that plots individual points by two numeric values to reveal relationships between them. Small multiplesA grid of small charts sharing the same axes, each showing a different slice of the data. Stacked bar chartA bar chart in which each bar is split into coloured segments that add up to the total.

T

Tick markA short mark on an axis at each interval, usually paired with a number or category label. TrendThe general direction a data series moves over time, ignoring short-term ups and downs.

V

VarianceThe average squared distance of values from the mean; a measure of spread.

X

X-axisThe horizontal axis, usually holding categories or time.

Y

Y-axisThe vertical axis, usually holding the measured value or quantity.

Words for describing what a chart shows

Beyond naming the parts, a few terms help you describe the data in words. Say whether a series increases, decreases, fluctuates or stays flat; name the peak (highest point) and trough (lowest point); describe the overall trend; point out any outliers; and, for two variables, describe the correlation as positive, negative or none. For a step-by-step method, see how to read a chart, and to pick the right chart in the first place, how to choose a chart.

Frequently asked questions

What are the basic chart terms?

The basic chart terms are the chart title, the x-axis and y-axis, the scale with its tick marks and labels, gridlines, axis titles, the legend, data series, data points and data labels. Together they describe every part of a standard chart.

What are the divisions on a chart axis called?

The divisions on an axis are called intervals, and each one is marked by a tick mark with a tick label. The full set of intervals and their values make up the axis scale. Lines extending the ticks across the plot are gridlines.

In a chart there is the axis and the what?

A chart usually has an axis, a legend, a title and a plot area. The axes give each point its position, the legend explains colours and symbols, and the plot area holds the data series themselves.

What is the difference between a legend and a data label?

A legend explains what each colour or symbol represents, while a data label prints the exact value of a single bar, slice or point. A chart can use both: the legend names the series and the labels give the numbers.

What is the difference between the x-axis and the y-axis?

The x-axis is the horizontal axis and usually shows categories or time. The y-axis is the vertical axis and usually shows the measured value. On a horizontal bar chart these roles are swapped.

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