Only use automatic unit conversion for simple units (not compound units)

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Disabled unit prefix conversions for compound units (e.g. `u"m/s"`) to avoid generating incorrect units. [#4583](https://github.com/MakieOrg/Makie.jl/pull/4583)
 - Fix polygon rendering issue of `crossbar(..., show_notch = true)` in CairoMakie [#4587](https://github.com/MakieOrg/Makie.jl/pull/4587).
 
 ## [0.21.16] - 2024-11-06

ReferenceTests/src/tests/unitful.jl

@@ -30,3 +30,16 @@ end
 
     st
 end
+
+@reference_test "Unit reflection" begin
+    # Don't swallow units past the first
+    f, a, p = scatter((1:10) .* u"J/s")
+    # Don't simplify (assume the user knows better)
+    scatter(f[1, 2], (1:10) .* u"K", exp.(1:10) .* u"mm/m^2")
+    # Only change prefixes of simple units, not compound units
+    scatter(f[2, 1], 10 .^ (1:6) .* u"W/m^2", (1:6) .* 1000 .* u"nm")
+    # Only change units/prefixes for simple units when adding more plots
+    scatter(f[2, 2], (0:10) .* u"W/m^2", (0:10) .* u"g")
+    scatter!((0:10) .* u"kW/m^2", (0:10) .* u"kg")
+    f
+end

src/dim-converts/unitful-integration.jl

@@ -20,13 +20,18 @@ base_unit(x::Period) = base_unit(Quantity(x))
 
 unit_string(::Type{T}) where T <: Unitful.AbstractQuantity = string(Unitful.unit(T))
 unit_string(unit::Type{<: Unitful.FreeUnits}) = string(unit())
-unit_string(unit::Unitful.FreeUnits) = unit_string(base_unit(unit))
+unit_string(unit::Unitful.FreeUnits) = string(unit)
 unit_string(unit::Unitful.Unit) = string(unit)
 unit_string(::Union{Number, Nothing}) = ""
 
 unit_string_long(unit) = unit_string_long(base_unit(unit))
 unit_string_long(::Unitful.Unit{Sym, D}) where {Sym, D} = string(Sym)
 
+is_compound_unit(x::Period) = is_compound_unit(Quantity(x))
+is_compound_unit(::Quantity{T, D, U}) where {T, D, U} = is_compound_unit(U)
+is_compound_unit(::Unitful.FreeUnits{U}) where {U} = length(U) != 1
+is_compound_unit(::Type{<: Unitful.FreeUnits{U}}) where {U} = length(U) != 1
+
 function eltype_extrema(values)
     isempty(values) && return (eltype(values), nothing)
 
@@ -159,22 +164,47 @@ function update_extrema!(conversion::UnitfulConversion, value_obs::Observable)
         imini = min(imini, mini)
         imaxi = max(imaxi, maxi)
     end
-    new_unit = best_unit(imini, imaxi)
+    # If a unit only consists off of one element, e.g. "mm" or "J", try to find
+    # the best prefix. Otherwise (e.g. "kg/m^3") use the unit as is and don't
+    # change it.
+    if is_compound_unit(imini)
+        if conversion.unit[] === automatic
+            new_unit = Unitful.unit(0.5 * Quantity(imini + imaxi))
+        else
+            return
+        end
+    else
+        new_unit = best_unit(imini, imaxi)
+    end
     if new_unit != conversion.unit[]
         conversion.unit[] = new_unit
     end
 end
 
 needs_tick_update_observable(conversion::UnitfulConversion) = conversion.unit
 
+# TODO: Convert the unit to rich text arguments instead of parsing the string
+# TODO: Could also consider UnitfulLatexify?
+function unit_string_to_rich(str::String)
+    chunks = split(str, '^')
+    output = Any[string(chunks[1])]
+    for chunk in chunks[2:end] # each chunk starts after a ^
+        pieces = split(chunk, ' ')
+        push!(output, superscript(string(pieces[1]))) # pieces[1] is immediately after ^
+        push!(output, join(pieces[2:end])) # rest is before the next ^
+    end
+    return rich(output...)
+end
+
 function get_ticks(conversion::UnitfulConversion, ticks, scale, formatter, vmin, vmax)
     unit = conversion.unit[]
     unit isa Automatic && return [], []
     unit_str = unit_string(unit)
+    rich_unit_str = unit_string_to_rich(unit_str)
     tick_vals = get_tickvalues(ticks, scale, vmin, vmax)
     labels = get_ticklabels(formatter, tick_vals)
     if conversion.units_in_label[]
-        labels = labels .* unit_str
+        labels = map(lbl -> rich(lbl, rich_unit_str), labels)
     end
     return tick_vals, labels
 end