Data items in the DIFFRN_SCAN_FRAME_AXIS category describe the
     settings of axes for particular frames.  Unspecified axes are
     assumed to be at their zero points.  If, for any given frame,
     nonzero values apply for any of the data items in this category,
     those values should be given explicitly in this category and not
     simply inferred from values in DIFFRN_SCAN_AXIS.