Allow X^n scaling in plots

[DanNixon]

This issue was originally TRAC 10580

Add the ability to change the X axis of a plot to x^n scale.

This can be done by implementing a new scale engine.

This forms part of #10564.

[DanNixon]

@martyngigg (2015-02-11T16:11:10):
There is a general requirement to be able plot in different units, as discussed here in http://trac.mantidproject.org/mantid/ticket/9722. This should not be implemented just for IDA. If this is still something that you'd like to tackle then we should discuss the design first.

---

@DanNixon (2015-02-11T16:49:06):
For now this is just going to deal with the plots inside the interface its self and is something I have done in http://trac.mantidproject.org/mantid/ticket/11036.

[martyngigg]

@ElliotAOram As this is now a general ticket I'm going to pass it to Matt as he has a bit more time available to spend on general things

[matthew-d-jones]

Following a conversation with Martyn:

• "n" will not be restricted to integer or positive values.
• A warning to "Ensure axis and data ranges are compatible with chosen axis scale" will be displayed in the GUI when the "power" scale type is selected.

It is hoped the warning will prompt users to check the axis or data range if the plot does not look like they expect. For example, it is possible to choose axis scales which diverge at 0, but are valid (and possibly useful) elsewhere.