Recipe request for logarithmic units

[anojo]

plot([2u"dB", 3u"dB"]) doesn't work for me. Quick tests like this suggest that logarithmic units aren't working, for either axis. Is it possible/easy to add a recipe for these?

[jw3126]

Interesting, thanks for reporting! The issue is, that we dispatch on Unitful.Quantity, however stuff measured in log units, apparently is not Quantity:

julia> using Unitful

julia> supertypes(typeof(2u"dB"))
(Gain{Unitful.LogInfo{:Decibel, 10, 10}, :?, Int64}, Unitful.LogScaled{Unitful.LogInfo{:Decibel, 10, 10}}, Number, Any)

julia> supertypes(typeof(2u"m"))
(Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, Unitful.AbstractQuantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, Number, Any)

This can probably be fixed by replacing Quantity by something like Union{AbstractQuantity, LogScaled} in this package. A PR would be welcome @anojo !

[briochemc]

I'm actually surprised that there is no parent abstract type for those:

julia> typejoin(typeof(1u"m"), typeof(1u"dB"))
Number

And I'm not sure there's a reason for that. Maybe worth asking the Unitful.jl peeps?

Otherwise, I think @jw3126's suggestion is the best and quickest solution, unless I'm missing something. Maybe we could add it a constant

const QuantityOrLogScaled = Union{AbstractQuantity, LogScaled}

and then dispatch on that?

(and also maybe it's worth upstreaming that up into Unitful?)

---

Also, side note: I realize we use Quantity throughout and not AbstractQuantity. Maybe we should loosen that up for future compatibility?

[anojo]

There's also that the function unit doesn't work (there's logunit instead), and that ustrip(u"dB", 2u"dB") doesn't seem to work either (but ustrip(2u"dB") does).

[briochemc]

OK so it looks like we need to think this through a bit more (and it will require a bit more work)!

[gustaphe]

Just a comment for this if you decide to go for it, a common construction is 1u"dB/m" which isa Quantity{Gain{Unitful.LogInfo{:Decibel, 10, 10}, :?, Int64}, 𝐋^-1, Unitful.FreeUnits{(m^-1), 𝐋^-1, nothing}}, so there is even more to think about.

[gustaphe]

I looked into this a little, and my conclusions so far:

• x::Level == 3u"dBm" can simply be plotted logarithmically using linear(x).
• x::Gain == 3u"dB" can slightly less simply be plotted logarithmically using uconvertp(NoUnits, x) (assuming power logarithm rather than root power logarithm here, from my experience that's pretty uncontroversial, but should maybe be a keyword argument).
• x::Quantity{<:Gain} == 3u"dB/m" is really tricky. It will rarely make sense to for instance convert this to a "linear" regime, as 0.01 /m ≠ -20 dB/m. So what we would want is to be able to save the "unit" dB/m on the axis, but we would then have to either feature request such a unit-like object from Unitful or roll our own, because in the Quantity object the Gain and the Unit is stored separately. There's a Unitful.MixedUnits type, but I haven't quite figured out how that enters into it.

https://github.com/gustaphe/UnitfulRecipes.jl/blob/logarithmic/src/logarithmic.jl