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Reading Bar Graphs

Bar charts (which are also called bar graphs and are similar to histograms) are charts that show (using bars of different heights) how data fall into different categories.  Bar charts are used instead of pie charts when the totals do not add up to 100% or when there are many categories.  (Many people like to use line graphs for everything, but line graphs are for things that change over time.  Bar charts are the correct graph to use when you are comparing categories.)

Bar charts take categorical data and place it in bars so that it’s easy to see which categories contain the most data.  The x-axis (across the bottom) will tell you the categories.  The y-axis (up and down) will tell you the numbers represented by the height of the bars.

When you look at a bar 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 categories.  The vertical axis (up the left side) should tell you what's being counted in each category and what each increment represents.  Look at the chart below:

  • Title: You can see it's about the completion of math homework.
  • x-axis: The categories are days of the week
  • y-axis: the units being counted are homework assignments and each gridline represents 10 assignments. You will often have to estimate exact numbers when bars fall above or short of the gridlines. Image removed.
Bar Chart

When you are asked to read a bar chart, you may be asked something very simple:

Example: About how many homework assignments were completed on Wednesday?

Find Wednesday on the x-axis.

See how high the bar goes.  It goes just past 20.  

Estimate: about 21 homework assignments were completed on Wednesday.

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

Example: According to the chart, the largest difference in homework completion was between which two consecutive days?

1. Find the key words.

"Difference" is the answer to a subtraction problem. 

"Consecutive" means one after the other.  So, you're only dealing with days that are right next to each other.

2. Read the graph and do the math.

Subtract each day’s score from the day before.  (Put the day with the largest amount first in the subtraction problems.) 

Monday - Tuesday = $28 - 27 = 1$

Tuesday - Wednesday = $27 - 20 = 7$

Thursday - Wednesday = $39 - 20 = 19$

Thursday - Friday = $39 - 18 = 21$

3. Look back to the problem to see what you're supposed to find: Find the largest difference.

 The biggest difference is between Thursday and Friday.

Some bar chart questions will ask you to find an average (add up all the datapoints, divide by the number of datapoints), the median (put the datapoints in order and find the middle one), or ask you something more specific (rather than the largest difference, it will ask you for the largest increase).  No matter what it asks, look at the graph carefully, write down the datapoints you need, and do the math.