A complex analysis generally proceeds within Ferret as a complex hierarchy of expressions: variables defined in terms of other variables defined in terms of other variables, etc., often containing many levels of transformation. When an error message such as "can only contour or vector a 2D region" occurs it may appear difficult to locate the reason for this message.
Several tools are available to help track down the source of such problems. The inter-related definitions of the variables in the hierarchy may be clarified through the use of the SHOW VARIABLE/TREE command (Ferret v6.85). For any given variable in the hierarchy use the STAT command to show its size and shape. A good starting point is often simply to edit the offending command line, replacing the PLOT, CONTOUR, VECTOR, etc. command with STAT and removing the qualifiers (if any) that control the graphics options. By repeatedly using STAT to examine the component variables of definitions one can quickly locate the source of the problem.