How do I change the axis proportions in 3D graphs?

You can change the aspect ratio for both 2D and 3D graphs on the Documents/Graphs: Settings tab of the Options dialog box [accessible by selecting Options from the Tools menu (classic menus) or by clicking Options on the Tools tab (ribbon bar)]. For 3D graphs, use the options in the 3D graphs axis proportion group.

For example, the default proportions of the axes in the graph above are 1:1:1 (i.e., the proportions of the X, Y, and Z sides are of equal length and form a regular cube). By changing the respective values, you can "flatten" (e.g., increase the proportions of the x- and y-axes compared to the z-axis: 1:1:.5)

or "stretch" (e.g., increase the relative length of just the z-axis: 1:1:2)

the 3D box in any direction. To change the axis proportions for a specific graph (i.e., locally), select the Graph: Layout tab in the Graph Options dialog box.