In our Data Visualization 101 series, we cover each chart type to help you sharpen your data visualization skills.
For a general data refresher, start here.
Line charts are one of the oldest forms of data visualization, and it’s not hard to see why. Whether you’re tracking the fluctuation of stock prices or the popularity of different wine varieties, they give you an accurate and quickly understandable assessment of the trend, acceleration, deceleration, and volatility of the data in question.
What Are Line Charts All About?
One of the most common charts in many fields, line charts display information as a series of data points connected by straight line segments on an X-Y axis. They are best used to track changes over time, using equal intervals of time between each data point (e.g., monthly earnings).
This line graph from the Krochet Kids intl. 2013 annual report shows the Net Worth (Currency-Neutral) for one of their female workers. =
Where It Came From
The first proto-line chart appeared somewhere in the 10th or 11th century in the appendix of a manuscript called De cursu per zodiacum (The Course of the Zodiac). Here, an unknown author plotted the planetary movements, depicted as cyclic lines on a spatial-temporal grid. (This was centuries before Copernicus and Galileo suggested the Earth rotated around the Sun.)
In this earliest of charts, we see time plotted against celestial latitude for the “classical planets” of Venus, Mercury, Saturn, Mars, Jupiter, the Sun, and the Moon:
The original line chart inventor remains a mystery, but by 1767 Swiss mathematician, physicist, philosopher, and astronomer Johann Heinrich Lambertwas used them regularly in his work. (He’s best remembered for proving that π was an irrational number.) Here, he graphs the evaporation of water vs. time:
Our old friend William Playfair is often credited with inventing the line chart, but even if that’s not true, he did play a large hand in making them successful.
In this data-rich example from Playfair’s later years, he compares the fluctuating numbers for stocks, revenue, expenditures, exports, the national debt, and the price of bread, clearly showing the effect of war on the British economy:
When to Use a Line Chart
If seeing the trend of your data is the goal, then this is the chart to use. Line charts show time-series relationships using continuous data (that which is measured and has a value within a range). They allow a quick assessment of acceleration (lines curving upward), deceleration (lines curving downward), and volatility (up/down frequency). They are excellent for tracking multiple data sets on the same chart to see any correlation in trends.
Tips for Creating Line Charts
With their innate simplicity, line charts are some of the easiest visualizations to get right. Add a little polish, and your data can really shine.
1. Include a Zero Baseline if Possible
Although a line chart does not have to start at a zero baseline, it should be included if possible. If relatively small fluctuations in data are meaningful (e.g., stock market data), you may truncate the scale to showcase these variances.
2. Label Lines Directly
This lets readers quickly identify lines and corresponding labels instead of referencing a legend.
3. Use the Right Height
Plot all data points so that the line chart takes up approximately two-thirds of the y-axis’ total scale.