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.

