Reading Line Graphs

Line charts (which are also called line graphs) are graphs that show (using lines that move up and down) how data change over time.  Line graphs are typically used when the x-axis (the horizontal line) is a change in time and the y-axis (vertical) is a change in some other number (amount, price, cost, etc.)

The main goal of a line graph is to show how data change over time, or how the changes in two different sets of data change over time. When you look at a line chart, first look at the title: What is this graph showing me?

Then, look at the axes.  The horizontal axis (along the bottom) should tell you the unit of time (years, months, decades, etc).  The vertical axis (up the left side) should tell you what change is being plotted (change in degrees, dollars, pounds, etc.)

Also, check to see if the graph has a single line or several lines.  If there is more than one line, figure out what each line represents.

Let's take the graph below, piece by piece:

• Title: You can see that this graph is about average rainfall in Los Angeles. 
• x-axis: This graph shows change by month.
• y-axis: They are using inches of rain.
• Line (or series) labels: There are two here.  The blue line represents the overall average rainfall.  The pink line represents the average in "El Nino Years."  You can expect questions about this graph to ask you to compare rainfall overall with rainfall in El Nino years (years where there's a oceanic even that tends to increase rainfall in Southern California). 

When you are asked to read a line graph, you may be asked something very simple:

Example: What is the overall rainfall average February?

Find February on the x-axis.

Find where the blue (Overall) line lands in February. It hits between 3.5 and 4, so about 3.8.  

Estimate: Los Angeles averages about 3.8 inches of rain in February.

Other times, you'll need to extrapolate data from the graph and then do some math to answer the question.

Example: According to the graph, during an El Nino Year, what is the average rainfall during the Winter Months (December-March)?

1. Find the key words.

"Average" means you should gather the data points and then find the average. 

"Winter Months" is defined in the question as December-March, so those are the data points you need.

2. Read the graph and do the math.

December:2.4

January:4.25

February:4.2

March:3.25

$2.4+4.25+4.2+3.25=14.1 \div 4=3.525$

Average Winter rainfall in Los Angeles in El Nino years is 3.525 inches.

Still other times, you might be asked to find trends in the data.

Example: Overall, which of the following statements captures the trends in rainfall in Los Angeles?

1. Rainfall in Los Angeles in El Nino years is significantly higher than rainfall in Los Angeles overall. 
2. Rainfall in Los Angeles in El Nino years is slightly lower than rainfall in Los Angeles overall.
3. Rainfall in Los Angeles in El Nino years is higher that rainfall in Los Angeles overall, especially during the beginning of the year.
4. Rainfall in Los Angeles in El Nino years is substantively the same as overall rainfall in Los Angeles, especially during the summer. 

Look at the two lines in the graph.  The pink line (El Nino Years) is almost always above the blue line (overall).  So, B and D are not correct.  Between A and C, C is a better answer for two reasons.  First, the differences are not huge, so the word "significantly" in A may not be completely true.  Moreover, the differences are definitely greater in January-April, so C captures the trends in the graph the best. 

Overall, line graphs are a great way to visually depict changes overtime and compare different changes overtime.  Whenever interpreting a line graph, just make sure to know what's going on on each axis and use your fingers to make sure that you read specific points correctly.  It's easy to look at the wrong line, or eyeball the wrong number on the y-axis, so read carefully.