cnLevelSelectionMode is set to ManualLevels (default is AutomaticLevels), which allows the user to set the minimum contour level (by using cnMinLevelValF), the maximum contour level (via cnMaxLevelValF), and the contour interval (with cnLevelSpacingF).
When cnLevelSelectionMode is set to EqualSpacedLevels (default is AutomaticLevels), cnMaxLevelCount can be used to set the number of contour levels. If the data array x is to be plotted, the contour interval is set by the following equation:
(max(x)-min(x))/cnMaxLevelCount = CI
The minimum contour level is then set to min(x)+CI, and the maximum contour level is set to max(x)-CI.
cnFillColors is also set here to explicitly set the colors to be used for the color shading.
The first frame creates a 2-box labelbar. The second frame creates a 1-box labelbar, by coloring the 2-box labelbar in the same color.
In a future release of NCL, we plan to add a special contour resource that handles constant fields more easily.