Units

AlgebraOfGraphics supports input data with units, currently Unitful.jl and DynamicQuantities.jl have extensions implemented.

Let's first create a unitful version of the penguins dataset:

julia

using AlgebraOfGraphics
using CairoMakie
using Unitful
using DataFrames

df = DataFrame(AlgebraOfGraphics.penguins())
df.bill_length = df.bill_length_mm .* u"mm"
df.bill_depth = uconvert.(u"cm", df.bill_depth_mm .* u"mm")
df.flipper_length = df.flipper_length_mm .* u"mm"
df.body_mass = df.body_mass_g .* u"g"
select!(df, Not([:bill_length_mm, :bill_depth_mm, :flipper_length_mm, :body_mass_g]))

first(df, 5)

5×7 DataFrame

Row	species	island	sex	bill_length	bill_depth	flipper_length	body_mass
	String	String	String	Quantity…	Quantity…	Quantity…	Quantity…
1	Adelie	Torgersen	male	39.1 mm	1.87 cm	181 mm	3750 g
2	Adelie	Torgersen	female	39.5 mm	1.74 cm	186 mm	3800 g
3	Adelie	Torgersen	female	40.3 mm	1.8 cm	195 mm	3250 g
4	Adelie	Torgersen	female	36.7 mm	1.93 cm	193 mm	3450 g
5	Adelie	Torgersen	male	39.3 mm	2.06 cm	190 mm	3650 g

When we plot columns with units, the units are automatically appended to the respective labels:

julia

spec = data(df) *
    mapping(:bill_length, :bill_depth, color = :body_mass) *
    visual(Scatter)
draw(spec)

Labels are separate from units, so we can relabel without affecting the unit suffixes:

julia

spec = data(df) *
    mapping(:bill_length => "Bill length", :bill_depth => "Bill depth", color = :body_mass => "Body mass") *
    visual(Scatter)
draw(spec)

We can choose a different display unit per scale via the unit scale property:

julia

draw(
    spec,
    scales(
        X = (; unit = u"cm"),
        Y = (; unit = u"mm"),
        Color = (; unit = u"kg")
    )
)

If we plot different units on the same scale, all the units have to be dimensionally compatible and will be auto-converted to the same unit.

julia

layer1 = data(df) * mapping(:bill_length, color = direct("Bill length")) * visual(Density)
layer2 = data(df) * mapping(:bill_depth, color = direct("Bill depth")) * visual(Density)

draw(layer1 + layer2)

AlgebraOfGraphics will complain if we try to plot dimensionally incompatible units on the same scale:

julia

layer1 = data(df) * mapping(:bill_length, color = direct("Bill length")) * visual(Density)
layer2 = data(df) * mapping(:body_mass, color = direct("Body mass")) * visual(Density)

draw(layer1 + layer2)

┌ Warning: Assignment to `layer1` in soft scope is ambiguous because a global variable by the same name exists: `layer1` will be treated as a new local. Disambiguate by using `local layer1` to suppress this warning or `global layer1` to assign to the existing global variable.
└ @ units.md:72
┌ Warning: Assignment to `layer2` in soft scope is ambiguous because a global variable by the same name exists: `layer2` will be treated as a new local. Disambiguate by using `local layer2` to suppress this warning or `global layer2` to assign to the existing global variable.
└ @ units.md:73
Merging the extrema of two subscales of the continuous scale X failed. This usually happens if two layers are combined which use data of different types for the same scale.
The incompatible extrema of the two scales were (32.1 mm, 59.6 mm) and (2700 g, 6300 g).
The error was: DimensionError: 2700.0 g and 32.1 mm are not dimensionally compatible.

In the next example, we make a facet plot with wide data, and even though there are two y axes, there's just one underlying Y scale being fit. Therefore, both columns get unit-converted:

julia

spec_wide = data(df) *
    mapping(:sex, [:bill_length => "Bill length", :bill_depth => "Bill depth"], layout = dims(1)) *
    visual(Violin)

draw(spec_wide)

Again, we can force a different display unit via the scale options.

julia

draw(spec_wide, scales(Y = (; unit = u"cm"), Layout = (; legend = false)))

Units ​

Units