7.04 Pie charts, bar charts and stacked area graphs

In today's information rich community it is important to get ideas across in an engaging and efficient form. You have probably come across many infographics in advertisements, magazines and presentations. An infographic is a collection of imagery, charts, and minimal text that strives to give an appealing and easy-to-understand overview of a topic.  Below is an infographic about space travel, specifically to the Moon, the information is represented in many different ways.

Infographic on visiting the Moon

To read infographics effectively you need skills in interpreting many different types of graphs. In this lesson we will look at interpreting a few familiar and unfamiliar charts that are often used.

Pie charts

A pie chart (pie graph or sector graph) is a special chart that uses a circle divided into slices(sectors) to show relative sizes of data. This chart is useful to display and compare parts of data that make up a whole, such as proportion of voters voting for particular political parties.

Practice question

Question 1

Segmented bar charts

A segmented bar chart is a graph where the bar represents the whole data set and the bar is divided into several segments to represent the proportional size of each category. It is very similar to a pie chart but with a bar replacing the circle.

Practice question

Question 2

Grouped bar charts

Grouped bar charts are useful when comparing two related data sets on the same graph.

Practice question

Question 3

Stacked area graphs

Stacked area graphs are similar to line graphs in that they both display data that changes over time. However, while a line graph shows how one variable changes over time (e.g. a product's sales figures from a particular shop), a stacked area graph is used to show changes in several variables that make up the total in the data being graphed (e.g. total sales figures across several stores in a chain). A stacked area chart shows how much each part contributes to the whole amount. 

The area between axes and lines are commonly emphasised with different colours or patterns.  

For example, the area chart below displays the total revenue made by a business across three products. The difference between the lines indicates the amount of revenue made by each product. For example, in the fourth month, Product B made $\$3000$$3000 ($\$9000-\$6000$$9000−$6000 ).

 

Worked example

Example 1

The revenues generated in thousands for a company from their four major products are shown in the area chart above.

(a) What was the monthly revenue generated by Product A in April?

Think: For the product at the bottom of the graph we need to just read the value of the line in April.

Do: In April, the line for Product A sits at $100$100. Hence, Product A produced $\$100000$$100000 revenue in April.

 

(b) What was the monthly revenue generated by Product C in May?

Think: We need to find the difference between the upper and lower lines for Product C in May.

Do: The top line for Product C sits at $450$450 and the bottom line sits on $300$300. As the difference is $150$150, Product C generated $\$150000$$150000 revenue in May.

(c) What percentage of the revenue in May was made by product C?

Think: We need to calculate the fraction of the revenue that Product C generated out of the total revenue in May and write this as a percentage.

Do: From part b) we have that Product C generated $\$150000$$150000 in revenue. Reading the top of the graph will give us the total revenue across the $4$4 products, hence, the total revenue in May is $\$550000$$550000.

$\text{Percentage revenue Product C in May}$Percentage revenue Product C in May	$=$=	$\frac{\text{Revenue for Product C}}{\text{Total revenue}}\times100%$Revenue for Product CTotal revenue​×100%
	$=$=	$\frac{150000}{550000}\times100%$150000550000​×100%
	$\approx$≈	$27.3%$27.3%

Practice question

Question 4