`DynamicQuantities.jl` extension

[Ickaser]

It would be lovely to have an extension like the existing Unitful extension for DynamicQuantities.jl, and I suspect that I could achieve that myself by copying the Unitful extension with some minor modifications to accommodate the differences in how DynamicQuantities handles things. Before I commit to that, though, I am curious if anyone (@gustaphe?) is seriously planning an overhaul of the unit handling for 2.0, in which case I would probably prefer to contribute some effort to that first.

[gustaphe]

When you say 2.0 do you mean Plots.jl 2.0?

I had not planned anything, but I might be out of the loop. Should I be?

As for copying the extension, perhaps it would be simpler and more robust to simply convert to Unitful quantities in the recipe? That way any future improvements to the extension work for both packages.

[Ickaser]

I do mean Plots.jl 2.0.

I had seen your comment in #4822 (comment) , and there are similar issues at #4741, #4750; plus #4453 would be nice.

The suggestion to map from DQ to Unitful would mean adding Unitful as a dependency of the extension, which seems weird and maybe difficult since DQ is probably the lighter package of the two; some things don't have one-to-one analogs and the extension namespace might be complicated, but if the dependency question has an easy answer then it would make sense to keep features in lockstep I think

[gustaphe]

It's maybe a question of measuring the impact of the packages - my feeling is that someone using DQ upon loading Plots will notice the impact of Plots, but not necessarily the impact of Unitful, but I always load both so maybe I don't know.
The really robust way to do this would be to have a lightweight common ground, but I'm not volunteering to write it.

I say give it a try. If you need to discuss something or if you figure out something we want for the Unitful extension, do let me know.

[t-bltg]

It's probably the most modern approach to use DynamicQuantities now. Things evolve and we should adapt.

A new extension, distinct from the Unitful one (but taking inspiration from it) can be written, yes.

[Ickaser]

While writing tests on #5158 , I had an idea, which I'll record here.

Currently the unit keyword for each axis is passed (eventually) to Unitful's ustrip, which gets called in the recipe for dealing with Unitful types. However, this doesn't prevent further quantities without units from getting plotted on the axis, which has always kinda bothered me.

Instead, the unit keyword argument could get used to divide all the values on that axis. As an example, passing a dimensionless yunit=1e4 would rescale the values on the axis by 1e-4 (and the "unit" in the axis guide tell you to multiply all quantities on that axis by 1e4 to read values). This would mean that we could share some of the underlying machinery between Unitful and DynamicQuantities. A Plots function like ustrip_dimensionless would get called on all on-axis quantities, and it would be enough for each unit extension to extend this method in an appropriate way.

Such a change might naturally go hand-in-hand with something like was proposed in #4453. It's entirely possible that inserting this in an appropriate place in the pipeline introduces new problems, but at first glance it seems to me like an elegant way to handle both packages with common machinery.