3-D Graphs 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. Figure 1. Mean Height of Brassica Rapa After First Week The independent variable in the 2D bar graph above is Solution Groups and the dependent variable is Height. Figure 1a. Amount of Money in Millions of Dollars Spent in Each Region of the Country in 1890 The independent variable in the 2D line graph above is Regions and the dependent variable is Amount of Dollars spent. But what if we have a second independent variable? 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. Figure 2. Height of Brassica Rapa Groups Over a Three Week Period The second independent variable added to the z-axis in the above graph is time (in Weeks). Figure 2a. The Amount of Money in Millions spent in Each Region of the Country Over the Sixty Year Period 1890 - 1950 The second independent variable added to the z-axis in the above graph is time (in Years). So when do you use a three-dimensional graph? 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. Sales Per Year by Department for Jeremy's Fashions 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. Remember: A three-dimensional graph is not used for decoration; it is used when you have two independent variables you need to plot. Make sure you include all variables in the title of your 3D graphs and don't ever forget axis labels and units. Copyright © 1998 Central Virginia Governor's School for Science and Technology Lynchburg, VA