Bar graphs are used to compare the relationship between two variables, usually independent and dependent, over a period of time. They are especially good for showing large changes over time of the variables. There are many different types of bar graphs, ranging from simple graphs to horizontal graphs and histograms. Multiple variables can also be compared using grouped bar graphs.
Simple Bar Graph

A simple bar graph can be used to compare a dependent and independent variable. In this case, the dependent variable simply shows the change in the independent variable. Because the dependent variable does not change, it can just be displayed as the unit of measurement for the independent variable. For example, if you use a bar graph to display the times at which an ice cream truck passes your house every day, the days of the week would be the dependent variable because you are not measuring their change. The time would be the independent variable.
Horizontal Bar Graph

Horizontal bar graphs are used in the same way as simple bar graphs, but the dependent variable is displayed on the horizontal axis. With the vertical axis at zero on the dependent scale, horizontal bar graphs are good for clearly showing negative and positive bars pointing in opposite directions. This graph would be good to chart the temperature outside every two weeks from December to May. The vertical axis would be freezing and the days that it was below freezing would have bars that pointed to the left of this axis and above freezing bars would point to the right.
Histogram

Histograms show the count of data points in groups. They display the frequency in which these points occur within the groups. In a histogram, the groups are shown on the horizontal axis and their frequency is shown on the vertical axis. You would use a histogram to show something like the exam scores of a class or grade of students.
Grouped Bar Graph

A grouped bar graph is used to show two or more independent variables and one dependent variable. The second independent variable represents different types of the independent variable Using different colors for each type is a simple way to distinguish them. The dependent variable should be displayed on the vertical axis and the grouped independent variables should be displayed on the horizontal axis. If you were graphing information about your exercising habits, one color for an independent variable could stand for weight lifting while another stood for cardiovascular exercise. The dependent variable could be amount of time you spent doing these exercises over a period of months.
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