7.1: Line Graphs
You're scanning some photographs for a customer. The customer is charged a $25 setup fee for the scanner and then 0.55 per scan. How much would the customer get charged for 8 scans? What about for 15 scans?
Watch This
First watch this video to learn about line graphs.
CK12 Foundation: Chapter7LineGraphsA
Then watch this video to see some examples.
CK12 Foundation: Chapter7LineGraphsB
Watch this video for more help.
Khan Academy Application problem with graph
Guidance
Before you continue to explore the concept of representing data graphically, it is very important to understand the meaning of some basic terms that will often be used in this concept. The first such definition is that of a variable . In statistics, a variable is simply a characteristic that is being studied. This characteristic assumes different values for different elements, or members, of the population, whether it is the entire population or a sample. The value of the variable is referred to as an observation, or a measurement. A collection of these observations of the variable is a data set .
Variables can be quantitative or qualitative. A quantitative variable is one that can be measured numerically. Some examples of a quantitative variable are wages, prices, weights, numbers of vehicles, and numbers of goals. All of these examples can be expressed numerically. A quantitative variable can be classified as discrete or continuous. A discrete variable is one whose values are all countable and does not include any values between 2 consecutive values of a data set. An example of a discrete variable is the number of goals scored by a team during a hockey game. A continuous variable is one that can assume any countable value, as well as all the values between 2 consecutive numbers of a data set. An example of a continuous variable is the number of gallons of gasoline used during a trip to the beach.
A qualitative variable is one that cannot be measured numerically but can be placed in a category. Some examples of a qualitative variable are months of the year, hair color, color of cars, a person’s status, and favorite vacation spots. The following flow chart should help you to better understand the above terms.
Variables can also be classified as dependent or independent. When there is a linear relationship between 2 variables, the values of one variable depend upon the values of the other variable. In a linear relation, the values of depend upon the values of . Therefore, the dependent variable is represented by the values that are plotted on the axis, and the independent variable is represented by the values that are plotted on the axis.
Linear graphs are important in statistics when several data sets are used to represent information about a single topic. An example would be data sets that represent different plans available for cell phone users. These data sets can be plotted on the same grid. The resulting graph will show intersection points for the plans. These intersection points indicate a coordinate where 2 plans are equal. An observer can easily interpret the graph to decide which plan is best, and when. If the observer is trying to choose a plan to use, the choice can be made easier by seeing a graphical representation of the data.
Example A
Select the best descriptions for the following variables and indicate your selections by marking an ‘ ’ in the appropriate boxes.
Variable  Quantitative  Qualitative  Discrete  Continuous 

Number of members in a family  
A person’s marital status  
Length of a person’s arm  
Color of cars  
Number of errors on a math test 
The variables can be described as follows:
Variable  Quantitative  Qualitative  Discrete  Continuous 

Number of members in a family  
A person’s marital status  
Length of a person’s arm  
Color of cars  
Number of errors on a math test 
Example B
Sally works at the local ballpark stadium selling lemonade. She is paid $15.00 each time she works, plus $0.75 for each glass of lemonade she sells. Create a table of values to represent Sally’s earnings if she sells 8 glasses of lemonade. Use this table of values to represent her earnings on a graph.
The first step is to write an equation to represent her earnings and then to use this equation to create a table of values.
, where represents her earnings and represents the number of glasses of lemonade she sells.
Number of Glasses of Lemonade  Earnings 

0  $15.00 
1  $15.75 
2  $16.50 
3  $17.25 
4  $18.00 
5  $18.75 
6  $19.50 
7  $20.25 
8  $21.00 
The dependent variable is the money earned, and the independent variable is the number of glasses of lemonade sold. Therefore, money is on the axis, and the number of glasses of lemonade is on the axis.
From the table of values, Sally will earn $21.00 if she sells 8 glasses of lemonade.
Now that the points have been plotted, the decision has to be made as to whether or not to join them. Between every 2 points plotted on the graph are an infinite number of values. If these values are meaningful to the problem, then the plotted points can be joined. This type of data is called continuous data . If the values between the 2 plotted points are not meaningful to the problem, then the points should not be joined. This type of data is called discrete data . Since glasses of lemonade are represented by whole numbers, and since fractions or decimals are not appropriate values, the points between 2 consecutive values are not meaningful in this problem. Therefore, the points should not be joined. The data is discrete.
Example C
The following graph represents 3 plans that are available to customers interested in hiring a maintenance company to tend to their lawn. Using the graph, explain when it would be best to use each plan for lawn maintenance.
From the graph, the base fee that is charged for each plan is obvious. These values are found on the axis. Plan A charges a base fee of $200.00, Plan C charges a base fee of $100.00, and Plan B charges a base fee of $50.00. The cost per hour can be calculated by using the values of the intersection points and the base fee in the equation and solving for . Plan B is the best plan to choose if the lawn maintenance takes less than 12.5 hours. At 12.5 hours, Plan B and Plan C both cost $150.00 for lawn maintenance. After 12.5 hours, Plan C is the best deal, until 50 hours of lawn maintenance is needed. At 50 hours, Plan A and Plan C both cost $300.00 for lawn maintenance. For more than 50 hours of lawn maintenance, Plan A is the best plan. All of the above information was obvious from the graph and would enhance the decisionmaking process for any interested client.
Vocabulary
Continuous data is data for which the plotted points can be joined, while discrete data is data for which the plotted points cannot be joined. A data set is a collection of observations of a variable. A variable is a characteristic that is being studied, with a continuous variable being a variable that can assume all values between 2 consecutive values of a data set, and a discrete variable being a variable that can only assume values that can be counted. Another way of classifying variables is by distinguishing between a qualitative variable , which is a variable that can be placed into specific categories according to some defined characteristic, and a quantitative variable , which is a variable that is numerical in nature and that can be ordered. When data is plotted on a coordinate grid, the independent variable is the variable represented by the values that are plotted on the axis, and the dependent variable is the variable represented by the values that are plotted on the axis.
Guided Practice
The local arena is trying to attract as many participants as possible to attend the community’s “Skate for Scoliosis” event. Participants pay a fee of $10.00 for registering, and, in addition, the arena will donate $3.00 for each hour a participant skates, up to a maximum of 6 hours. Create a table of values and draw a graph to represent a participant who skates for the entire 6 hours. How much money can a participant raise for the community if he/she skates for the maximum length of time?
Answer:
The equation for this scenario is , where represents the money made by the participant, and represents the number of hours the participant skates.
Numbers of Hours Skating  Money Earned 

0  $10.00 
1  $13.00 
2  $16.00 
3  $19.00 
4  $22.00 
5  $25.00 
6  $28.00 
The dependent variable is the money made, and the independent variable is the number of hours the participant skated. Therefore, money is on the axis, and time is on the axis as shown below:
A participant who skates for the entire 6 hours can make $28.00 for the "Skate for Scoliosis" event. The points are joined, because the fractions and decimals between 2 consecutive points are meaningful for this problem. A participant could skate for 30 minutes, and the arena would pay that skater $1.50 for the time skating. The data is continuous.
Interactive Practice
Practice

What term is used to describe a data set in which all points between 2 consecutive points are meaningful?
 discrete data
 continuous data
 random data
 fractional data

What type of variable is represented by the number of pets owned by families?
 qualitative
 quantitative
 independent
 continuous

What type of data, when plotted on a graph, does not have the points joined?
 discrete data
 continuous data
 random data
 independent data
 Select the best descriptions for the following variables and indicate your selections by marking an ‘ ’ in the appropriate boxes.
Variable  Quantitative  Qualitative  Discrete  Continuous 

Men’s favorite TV shows  
Salaries of baseball players  
Number of children in a family  
Favorite color of cars  
Number of hours worked weekly 
You are selling your motorcycle, and you decide to advertise it on the Internet on Walton’s Web Ads. He has 3 plans from which you may choose. The plans are shown on the following graph. Use the graph and explain when it is best to use each plan.
 When would it be best to use Plan A?
 When would it be best to use Plan B?
 When would it be best to use Plan C?

What is the dependent variable in the following relationship? The time it takes to run the 100 yard dash and the fitness level of the runner.
 fitness level
 time
 length of the track
 age of the runner
 If the relationship in question 8 were graphed on a coordinate grid, what variable would be on the x axis?
 If the relationship in question 8 were graphed on a coordinate grid, what variable would be on the y axis?
Image Attributions
Description
Learning Objectives
Here you'll learn the difference between continuous data and discrete data as it applies to a line graph. You'll also learn how to represent data that has a linear pattern on a graph and how to solve problems with line graphs.