rejecting trans argument on scales with fixed trans

[TyberiusPrime]

With regard to our discussion in #254,
this is how I would solve the 'fixed-trans-scales-must-raise-on-trans-paramter != their-trans' goal.

All it needs is a mixin base class - the scales were using multiple inheritance already (scale_x_datetime),
so no additional magic needs to be understood (unlike e.g. a solution with a class decorator) and the boilerplate is minimal.

Not sure if you want the base class with @document.

[has2k1]

I was reluctant to introduce a mixin class because, I wanted to show a warning and a mixin would be too much just for that function. I think only the documentation needs to change.

[TyberiusPrime]

I believe warnings are for 'this might be wrong or not, the lib can't tell'-situations, while
obviously wrong (as in "you have told me contradictory things to do") should raise an Exception.

I can see two other straight forward implementations, either with a class decorator (magic), or a
flag on the transformed-scale classes (e.g. use '_fixed_trans' instead of _trans) which scale_continuous then interprets as 'warn/raise on trans'.

(Philosophically, having two ways to define transformations, scale_continous(trans=) and scale_trans is unpythonic - but nothing we can do about that).

[has2k1]

use '_fixed_trans' instead of _trans

That would work, but I think there is a better way.

[TyberiusPrime]

A separate flag?

You might get away with having the test in scale_continuous if _trans != Identity (or waiver...) But it is not clear and poses a restriction for all child classes not just those that opt in.

[has2k1]

Thanks for raising and exploring this issue, I have resolved it in 0c9f85b with a warning when the trans for non-identity scale is set to a different transformation.