void zlab (on_off, labstr)  LABSET.C Level 1
int on_off  = 0 to turn off the z axis string labels (default). 
= 1 to turn on the z axis string labels.
char *labstr[ ]  An array of pointers to the individual string labels for each successive tick mark. 
zlab() turns on or off the z axis string label option and operates like xlab(). When it is on, each string is positioned at a tick mark in the same way as numerical labels. There must be at least as many string elements in the array as there are x tick marks. You must ensure that the string will fit between the tick marks. See the example BARTEST.C to see how to use string labels.