There is no y axis because the files are relative to one another. Here is the information from the help section:
The Read/Write Performance by File chart displays files with latency readings that have reached a Warning or Critical level. Bigger bars reflect both higher latency (above or significantly above critical) and possibly sustained high latency over extended periods of time. If this chart is empty, it means that file latencies have not crossed a warning or critical threshold during this time period.
Note: Latency values are specific to each environment -- what may be high values in one company, or even one file for that matter, may be acceptable in others. It is good practice to determine the latency values for your files when performance is acceptable and set warning and critical thresholds accordingly. To change the warning and critical threshold levels, go to Options (Monitor Setup tab) > I/O Configuration (I/O Thresholds tab).
Please let me know if you have any other questions.