The 'maximum' metrics require a higher Statistics Level, namely 4.

As you find out in the PerformanceManager documentation.

You can, as an alternative, take the maximum value of the averages over a period of time.

Don't forget that the Historical Interval 1 (past day) has in any case aggregated the values to 5 minute intervals.

See also my PowerCLI & VSphere Statistics – Part 1 – The Basics post.