update y axis label based on unit type

[cshanahan1]

Check unit type (e.g flux density, surface brightness, counts, etc) for generating display label for the y axis in spectral viewer. Previously it was hard coded to always display 'flux density' no matter the input unit.

Closes #2701

[pllim]

"counts" is very ambiguous by STScI standard. I don't think this is correct.

[kecnry]

given that the unit is also in the label, I think this is probably ok?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fb6d841) 90.83% compared to head (04a29ed) 90.84%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2703   +/-   ##
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  Coverage   90.83%   90.84%           
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  Files         163      164    +1     
  Lines       21502    21524   +22     
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+ Hits        19532    19553   +21     
- Misses       1970     1971    +1     

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[pllim]

I think it is also a matter of how consistent we want to be w.r.t. y_unit.physical_type that you get from astropy.units. If consistency isn't a concern, then what you have is fine.

[pllim]

Do we need tests?

[pllim]

I think you can access self.figure.axes[1].label and check that the desired flux unit type is there, no?

[cshanahan1]

added tests

[pllim]

Thanks! Won't catch edge cases like u.electron (without per second) but we can handle those as need arises because there are unlimited combos possible.

[pllim]

devdeps failure is unrelated (glue-viz/glue-jupyter#421)

[kecnry]

Definitely an improvement and we can always iterate or move to somewhere more general (for all axes in all viewers) down the road. Thanks for finding and fixing this!

[pllim]

LGTM. Thanks!