Chart and Graph Glossary: Key Terms

100% stacked bar chart: A stacked bar chart in which every bar is scaled to the same full height, so each segment shows a share of its bar’s total. See types of bar graphs.
Axis: A reference line on a chart that carries labels or a scale. A bar graph has a category axis and a value axis.
Bar graph (bar chart): A chart that uses bars whose lengths are proportional to values in order to compare categories. Try the bar graph maker.
Baseline: The line from which bars start. On a bar graph it should be zero.
Bin: A range of values used to group numbers in a histogram, such as “10 to under 20”. Also called a class interval.
Bin width: The size of each bin in a histogram. See how to choose a bin width.
Categorical data: Data that places items into groups, such as colors or countries, rather than measuring them on a number scale. Usually charted with a bar graph.
Category: One of the groups being compared in a bar graph, shown as one bar or one group of bars.
Clustered bar chart: Another name for a grouped bar chart. Excel uses this term.
Continuous data: Numerical data that can take any value in a range, such as height, time or temperature. Usually charted with a histogram.
Cumulative percentage: The running total of percentages as you move through the categories, ending at 100%. It is the line on a Pareto chart.
Data label: A number written on or beside a bar or point to show its exact value.
Density: A histogram measure equal to count ÷ (number of values × bin width). It makes the total area of the bars equal to 1.
Discrete data: Numerical data that takes separate values, usually counts, such as the number of siblings.
Donut chart: A pie chart with a hollow center, which can show the total. See the pie chart maker.
Double bar graph: A grouped bar graph with exactly two series. The term is common in school maths.
Frequency: How many times a value or category occurs in the data.
Frequency table: A table listing categories or bins alongside their frequencies. The histogram maker builds one from your raw numbers.
Gridline: A light line across the plot area at each tick mark, which helps readers read values against the scale.
Grouped bar chart: A bar chart with two or more bars side by side for each category, one per series. Also called clustered, or (with two series) a double bar graph. See the grouped bar chart maker.
Histogram: A chart that shows how one continuous number is distributed by cutting it into equal-width bins and drawing a bar for the count in each. Its bars touch. See bar graph vs histogram.
Horizontal bar graph: A bar graph with horizontal bars: categories run down the left and values along the bottom. Best for long labels. See the horizontal bar graph maker.
Interquartile range (IQR): The third quartile minus the first quartile (Q3 − Q1). It measures the spread of the middle half of the data and is used by the Freedman–Diaconis bin rule.
Legend: A key that says which color or pattern stands for which series or category.
Line graph: A chart that connects values in order with a line, used to show change over a sequence such as time. See the line graph maker.
Mean: The average: the sum of the values divided by how many there are.
Median: The middle value when the data is sorted. Half the values are below it and half above.
Outlier: A value that lies far from the rest of the data. It can stretch a histogram’s bins or an axis.
Pareto chart: A bar chart sorted from largest to smallest, with a line showing the cumulative percentage. It is used to find the “vital few” causes behind most of a total. See the Pareto chart maker.
Pie chart: A circle divided into slices whose angles are proportional to each part’s share of the whole. See bar chart vs pie chart.
Quartile: One of three values that split sorted data into four equal-sized groups. Q1 is the 25th percentile, Q2 the median, and Q3 the 75th percentile.
Relative frequency: A frequency divided by the total number of values, often written as a fraction or percentage.
Scale: The numbers and steps marked along an axis. A good scale has even steps and, for bars, starts at zero.
Series: A set of values plotted together, such as one column in your data table. Each series in a grouped or stacked chart has its own color and legend entry.
Stacked bar chart: A bar chart in which each bar is split into segments, one per series, that add up to the bar’s total. See the stacked bar chart maker.
Standard deviation: A measure of how spread out values are around the mean. The sample standard deviation divides by n − 1 and is used by Scott’s bin rule.
Tick mark: A small mark on an axis at a scale value, usually paired with a label.
Value axis: The axis that carries the scale of the measured quantity. On a vertical bar graph it is the vertical axis.
Vital few: In a Pareto chart, the small number of categories that account for most of the total.
Zero baseline: Starting the value axis at zero so that bar lengths are proportional to values. See bar graph best practices.