Graphing — Line Plots and Bar Graphs

A graph turns a list of numbers into a picture you can read at a glance. Line plots stack X marks above a number line to show how often each value appears. Bar graphs use the height of bars to compare totals across categories.

Line Plots

• A line plot has a number line at the bottom with each possible value labeled in order.
• Above each value, an X mark is drawn for every data point that equals that value.
• The number of Xs above a value is called the frequency — how many times that value appears.
• To find the total number of data points, count every X in the whole plot.

Bar Graphs

• A bar graph has two axes: a category axis (often the x-axis) and a value axis (often the y-axis).
• Each bar stands above one category. The height of the bar shows the value for that category.
• The scale on the value axis tells you how much each grid line is worth — for example, every line could equal 2.
• To read a bar, find the top of the bar and slide across to the scale to read its value.

Asking Questions of a Graph

• Most or fewest: find the tallest or shortest bar, or the value with the most or fewest Xs.
• How many in all: add the values of every bar, or count every X in the line plot.
• How many more: subtract the smaller value from the larger value to find the difference.
• Always check the scale before answering — a bar that reaches the 4th grid line stands for 8, not 4, if each line is worth 2.

Choosing Between a Line Plot and a Bar Graph

• Use a line plot when the data are numbers and you want to see how often each number appears.
• Use a bar graph when the data fall into named categories like fruits, sports, or months.
• Both kinds of graph need a clear title and labeled axes so a reader can understand the data.