How do I define a custom selection of levels for a contour plot or surface plot?

The primary controls for defining contour intervals for 3D surface or contour plots are accessed in the Surface Specifications dialog box, accessed by clicking the Surface specs button on the Plot: Fitting tab of the Graph Options dialog box. (Display the Graph Options dialog box by double-clicking on the outer background area of the graph.)

Number of contours/levels

The number of color levels placed on the graph is determined by default by the number of minor tickmarks spanned by the surface on the Z axis of the plot (see Axis: Minor Units tab). This default option is in the Get shading levels from box near the top of the Surface Specifications dialog box (available by clicking the Surface specs button on the Plot: Fitting tab). Note that other options for determining the number of contour levels to place on the graph include major tickmarks (see Axis: Major Units tab), custom tickmarks (see Axis: Custom Units tab), and combinations of custom with major or minor tickmark options.

Specifying intervals

To control the numeric values that divide contours, use the Custom Units tab of the Graph Options dialog box to specify as custom tickmarks the values corresponding to boundaries between contours. Then select Custom tickmarks from the Get shading levels from box near the top of the Surface Specifications dialog box (available by clicking the Surface specs button on the Plot: Fitting tab).