Library /sys$common/syshlp/pca$help.hlb PLOT, Data-kind qualifiers, /PHYSICAL_IO_COUNT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Tallies the total number of physical I/O operations (reads plus
    writes) along the horizontal axis. To use this qualifier, you
    must have collected I/O services data.