Choosing the right graph for the data
Match the data type to the graph: bar for categories, line for change over time, scatter for two numeric variables.
- Categorical vs. numeric data: You must first tell groups (categories) apart from measured numbers to know whether bars or points fit.
- Independent vs. dependent variable: Knowing which variable you changed and which you measured tells you what goes on each axis.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Bar charts compare separate groups, line graphs show change over time, and scatter plots show how two numeric variables relate.
A student measures one volunteer's heart rate once every minute for 20 minutes after exercise to see how it recovers. Which graph best shows this?
Reviewed- A.Bar chart
- B.Line graph
- C.Scatter plot of two unrelated people
- D.Pie chart
Show the worked solution ▾
Answer: B. Line graph
- Step 1: Name the variables: The bottom axis is time (minutes), and the side axis is the measured heart rate.
- Step 2: Match time to a graph: One measure changing as time passes is shown with a line graph, which connects the points in time order.
Why it's right: The data is one measurement changing over time, which is exactly what a line graph is for.
- A: A bar chart compares separate groups, but here time runs continuously.
- C: There is one person measured over time, not two variables across people, so a scatter plot does not fit.
- D: A pie chart shows parts of a whole, not change over time.
Aligned to BI 2.1: graph selection · reading level ~grade 9
Researchers measure each of 30 people's height and their lung volume and want to see whether taller people tend to have larger lung volume. Which graph fits?
Reviewed- A.Bar chart of average height
- B.Line graph over time
- C.Scatter plot of height versus lung volume
- D.Pie chart of heights
Show the worked solution ▾
Answer: C. Scatter plot of height versus lung volume
- Step 1: Count the numeric variables: Height is a measured number and lung volume is a measured number, so there are two numeric variables, one per person.
- Step 2: Match to a graph: Two numeric variables you want to compare are plotted as points, one point per person: a scatter plot.
Why it's right: Two numeric measurements per person, checked for a relationship, are shown with a scatter plot.
- A: A bar chart compares groups and would hide the person-by-person relationship.
- B: Nothing here changes over time, so a line graph does not fit.
- D: A pie chart shows parts of a whole, not a relationship between two measurements.
Aligned to BI 2.1: graph selection · reading level ~grade 9
- A health report comparing five clinics' wait times uses bars; a chart of one patient's temperature across a day uses a line.
Fill these in as you work through the lesson.
- Categorical data (labels or groups, not amounts):
- Numeric data (measured amounts you can put in order):
- Trend over time (how one measure changes as time passes):
- Relationship (how two measured things move together):
Use a chart to compare separate groups, a graph to show change over time, and a plot to show how two numeric variables relate.
- A study records average resting heart rate for four sports teams. Which graph fits, and why?
- You measure one runner's heart rate every minute for 20 minutes. Which graph fits, and why?
- You measure each person's height and their lung volume. Which graph shows whether they relate?
A class records the favorite blood-type fact for 5 separate clubs, then later records one club's CO2 reading every 2 minutes for an hour. Decide which graph fits each dataset and explain what the axes would be.