When viewing the Real-time or History pages, there are times when the "Datafile I/O" tab is empty. Sometimes it shows entries only when selecting "Last 24 hours" and the rest does not return any data at all.
"Datafile I/O" tab displays a listing of datafiles, in the current dimension drill-down context, on which the most I/O resource-demanding data activities occurred at a particular time.
If you do not see any entries but you are certain that there were I/O activities, then try changing the "Data Interval" on the upper right hand corner to a more precise value like "1 minute" or "15 minutes". You can also scroll the graph horizontally to a time when I/O activity occurred.