Be sure you are familiar with two-dimensional graphs before you explore this section. As you probably know, 2D graphs have two variables - an independent plotted on the x-axis, and a dependent plotted on the y-axis.
The independent variable in the 2D bar graph above is Solution Groups and the dependent variable is Height.
The independent variable in the 2D line graph above is
Regions and the dependent variable is Amount of Dollars spent.
When you need to plot a third variable, you need a 3-dimensional graph with a third axis, the z-axis. The dependent variable is still on the y-axis and the original independent variable is still the on the x-axis, but now the second independent variable is on the z-axis.
The second independent variable added to the z-axis in the above graph is time (in Weeks).
The second independent variable added to the z-axis in the above graph is time (in Years).
When you have two independent variables and one dependent variable. For example, let's say you have a department store and you want to graph yearly sales. You could make a two-dimensional bar graph with the independent variable (years) plotted on the x-axis and the dependent variable (sales in $) plotted on the y-axis. BUT let's get more advanced and add a second independent variable (different departments) plotted on the z-axis.
In this case, creating a 3D bar graph by adding the second independent variable (Department) to the z-axis, breaks the sales data into sub-categories and shows each department's contribution to the total yearly sales. In all of the examples above, the second independent variable had sub-categories; discrete units of the variable. In another type of 3D graph, the surface plot, the second independent variable has continuous units.
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