How Do Line Graphs Work?
Connecting data points with lines to reveal trends, patterns, and changes over time.
Showing Change Over Time
A line graph plots data points on a grid and connects them with a line. It's the best type of graph for showing how something changes over time: temperature throughout the day, a plant's height each week, or a city's population over decades. The line's direction tells the story at a glance — going up means increasing, going down means decreasing, and flat means staying the same.
Reading a Line Graph
The x-axis (horizontal) usually shows time intervals — days, months, years. The y-axis (vertical) shows the quantity being measured — temperature, height, dollars, population. Each data point sits where its time value and quantity value intersect. The line connecting the points shows the trend — the overall direction of change. A steep line means rapid change; a gentle slope means gradual change.
Multiple Lines for Comparison
One of the most powerful features of line graphs is that you can plot multiple lines on the same graph. Want to compare rainfall in three cities over a year? Use three different colored lines on one graph. This makes differences and similarities jump out visually — you can instantly see which city gets the most rain, when the rainy season hits, and how patterns differ.
Line Graphs vs. Bar Graphs
Both display data, but they serve different purposes. Bar graphs compare separate categories (favorite sports, sales by product). Line graphs show continuous change, especially over time. If you want to show Monday's temperature vs. Tuesday's vs. Wednesday's as a trend, use a line graph. If you want to compare total rainfall in different cities as separate amounts, a bar graph is better.
Florence Nightingale, the famous nurse, was also a pioneering data visualizer. In the 1850s, she created innovative circular statistical charts (now called "Nightingale rose diagrams") to show that more British soldiers were dying from preventable diseases than from battle wounds during the Crimean War. Her compelling visual arguments convinced the British government to improve hospital sanitation — saving thousands of lives. She proved that the right graph can literally change the world.
Last reviewed: April 2026