Text: How to Read Graphs

Line graphs and bar graphs are both visual ways of representing two or more data sets and their interrelation. In other words, graphs are pictures that show you how one thing changes in relation to another. Learning to read graphs properly is a matter of interpreting which pieces of information go together.

Steps for Reading Graphs

1. Identify what the graph represents. Most graphs will have a clearly labeled x-element, spaced along the graph’s horizontal axis, and a clearly labeled y-element, spaced along the graph’s vertical axis.

  • The graph’s title should also tell you exactly what it’s about.
Graph: Returned salmons in the river in 2013. Vertical line is labeled "y-axis (quantity)" and is circled in pencil. Increments start at 6000 at the bottom, with one line for every increasing 1000 up to 15,000 at the top. Horizontal line is labeled “x-axis (month)” and is circled in pencil. Every month of the year is marked, beginning with January at the left and ending with December on the right. Points appear at the appropriate intersection of every month/ number pairing, and everything under the implicit line connecting the dots is shaded blue. The graph shows a peak of 15,000 salmon in March, a trough of 6000 salmon in June, along with a general downward trend towards the end of the year.

2. Check the scale for each graph element. This applies to both line graphs and bar graphs.

  • For example, if you’re looking at a graph that shows how many salmon returned to a given stream over the period of a few months last year, each increment along the graph’s y-axis might represent hundreds, thousands or tens of thousands of salmon returning; you won’t know which scale numbers apply until you check the graph.
Close up photo of same graph, emphasizing the circled "y-axis (quantity)." A pencil sits right next to the phrase.

3. Locate the graph element you want information on.

  • For example, you might want to know how many salmon returned to the stream in question in August of last year. So you’d read across the graph’s horizontal axis until you find “August.”
  • Time elements, such as days, weeks, months or years, are almost always listed along the horizontal (“x”) axis. Quantity measurements are almost always listed along the vertical (“y”) axis.
Photo close-up on same graph, focusing on the horizontal axis. A pen is lined up under the indicator for August, which is underlined in pencil.

4. Read directly up from “August” until you find a dot or a slanting line, on a line graph, or the top of a bar for a bar graph. Then read straight across to the left until you hit the graph’s labeled y-axis. Whatever quantity that line intersects with is the measurement for salmon return in August.

  • So if you read up to the dot, line or top of the bar for salmon in August then read across to the left and hit “10,000,” you know 10,000 salmon returned in August. If you hit a point between any two labeled graph increments, you have to estimate based on where you land between the 2 increments. For example, if you hit a point halfway between 10,000 and 15,000, you can safely estimate that the correct number is about 12,500.
Photo close-up of same graph. A marker points towards the 10,000 line on the vertical axis; a pencil points towards "August" on the horizontal axis. A dotted line has been drawn on the graph to show where the two items intersect on the graph, at a point.


Final Tips

  • Line graphs give you 1 piece of explicit information that bar graphs don’t. The steepness of the line connecting each data point on the graph (that is, each dot) indicates the rate of change. A steeply descending line, for example, would show that salmon returns dropped abruptly from 1 month to the next. But a slowly ascending line represents a gradual increase.
  • If the graph correlates more than 2 elements, the extra data sets are usually assigned to the x-, or horizontal axis. Often, the additional data sets are graphed in another color to avoid confusion. So if you want to compare the salmon returns over the same months for more than 1 year at a time, you could graph each year’s return on the same graph, but in a different color.