You cannot set it directly but it can read maximum and minimum and adjust range automatically.
Or you can draw a line of maximum and minimum line for the given time range.
All this is detailed in Value Scale section in PI Coresight 2015 User Guide Page 20
PI Coresight is not flexible for setting minimum and maximum. (Not only adhoc trend but also normal trend etc...)
I want to share one thing that can set minimum and maximum.
Of cause PI tag's attribute (zero, span) changing works.
But formula also works. Set minimum and maximum in formula like following.
PI Coresight can show this formula attribute with minimum 0, maximum 200 for sinusoid tag.
Sorry this is not settings for adhoc trend.