| 
            plslabelfunc
          ( | label_func , | 
| label_data ); | 
      This function allows a user to provide their own function to provide
      axis label text.  The user function is given the numeric value for a
      point on an axis and returns a string label to correspond with that
      value.  Custom axis labels can be enabled by passing appropriate
      arguments to plenv, plbox,  plbox3 and similar functions.
    
label_func
          (PLLABEL_FUNC_callback
            This is the custom label function.  In order to reset to the
            default labelling, set this to NULL.
            The labelling function parameters are, in order:
            
axis
                
                    This indicates which axis a label is being requested for.
                    The value will be one of PL_X_AXIS,
                    PL_Y_AXIS or
                    PL_Z_AXIS.
                  
value
                This is the value along the axis which is being labelled.
label_text
                
                    The string representation of the label
                    value.
                  
length
                
                    The maximum length in characters allowed for
                    label_text.
                  
label_data
          (PLPointer
            This parameter may be used to pass data to the
            label_func function.
          
This function is used in example 19.