Make singular limits nonsingular (#634)

Fixes #590 

Calculation of colors and markersizes failed when the limits were
singular, so with this PR they are expanded to 0, v or v, 0 depending on
if v is below or above zero, and to 0, 1 if v is zero.

```julia
f = Figure(size = (500, 600))
fg = draw!(f[1, 1], pregrouped([1:3], [1:2], [ones(3, 2)]) * visual(Heatmap))
colorbar!(f[1, 2], fg)
fg2 = draw!(f[2, 1], pregrouped([1:3], [1:2], [zeros(3, 2)]) * visual(Heatmap))
colorbar!(f[2, 2], fg2)
fg3 = draw!(f[3, 1], pregrouped([1:3], [1:2], [-2 .* ones(3, 2)]) * visual(Heatmap))
colorbar!(f[3, 2], fg3)
fg4 = draw!(f[1, 3], mapping(1:5, 1:5, markersize = ones(5)) * visual(Scatter))
legend!(f[1, 4], fg4)
fg5 = draw!(f[2, 3], mapping(1:5, 1:5, markersize = zeros(5)) * visual(Scatter))
legend!(f[2, 4], fg5)
fg6 = draw!(f[3, 3], mapping(1:5, 1:5, markersize = -2 .* ones(5)) * visual(Scatter))
legend!(f[3, 4], fg6)
f
```

<img width="490" alt="image"
src="https://github.com/user-attachments/assets/c46816e5-6ec9-4d17-858c-08f3bd0e36b2"
/>

Author: jkrumbiegel

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Added better error messages for the common case of failing to construct single element NamedTuples in calls like `draw(axis = (key = value))` [#630](https://github.com/MakieOrg/AlgebraOfGraphics.jl/pull/630).
+- Fixed bug when color or markersize mappings had singular limits by expanding the limits to `(0, v)`, `(-v, 0)` or `(0, 1)` [#634](https://github.com/MakieOrg/AlgebraOfGraphics.jl/pull/634).
 
 ## v0.10.0 - 2025-03-30
 

src/algebra/layers.jl

@@ -496,10 +496,35 @@ end
 
 full_rescale(data, aes, scale::CategoricalScale) = rescale(data, scale)
 
+function nonsingular_colorrange(scale::ContinuousScale)
+    props = scale.props.aesprops::AesColorContinuousProps
+    cr = @something(props.colorrange, scale.extrema)
+    nonsingular_limits(cr)
+end
+
+# expand singular limits to (0, v) or (v, 0) if singular value v is nonzero
+# or to (0, 1) if it is, which should be easier to read than choosing a middle
+# value like it's done in Makie with axis limits
+function nonsingular_limits(r)
+    if r[1] == r[2]
+        if r[1] == 0
+            return (r[1], oneunit(r[1]))
+        else
+            if r[1] < 0
+                return (r[1], false * r[1])
+            else
+                return (false * r[1], r[1])
+            end
+        end
+    else
+        return r
+    end
+end
+
 function full_rescale(data, aes::Type{AesColor}, scale::ContinuousScale)
     props = scale.props.aesprops::AesColorContinuousProps
     colormap = Makie.to_colormap(@something(props.colormap, default_colormap()))
-    colorrange = Makie.Vec2(@something(props.colorrange, scale.extrema))
+    colorrange = Makie.Vec2(nonsingular_colorrange(scale))
     lowclip = Makie.to_color(@something(props.lowclip, first(colormap)))
     highclip = Makie.to_color(@something(props.highclip, last(colormap)))
     nan_color = Makie.to_color(@something(props.nan_color, RGBAf(0, 0, 0, 0)))
@@ -514,9 +539,9 @@ function full_rescale(data, aes::Type{AesColor}, scale::ContinuousScale)
     )
 end
 
-function full_rescale(data, aes::Type{AesMarkerSize}, scale::ContinuousScale)
+function full_rescale(data, ::Type{AesMarkerSize}, scale::ContinuousScale)
     props = scale.props.aesprops::AesMarkerSizeContinuousProps
-    values_to_markersizes(data, props.sizerange, scale.extrema)
+    values_to_markersizes(data, props.sizerange, nonsingular_limits(scale.extrema))
 end
 
 full_rescale(data, aes::Type{<:Union{AesContourColor,AesABIntercept,AesABSlope}}, scale::ContinuousScale) = data # passthrough, this aes is a mock one anyway
@@ -568,7 +593,7 @@ function to_entry(P::Type{Heatmap}, p::ProcessedLayer, categoricalscales::Dictio
     else
         color_attributes = dictionary([
             :colormap => @something(scale.props.aesprops.colormap, default_colormap()),
-            :colorrange => @something(scale.props.aesprops.colorrange, scale.extrema),
+            :colorrange => nonsingular_colorrange(scale),
             :nan_color => @something(scale.props.aesprops.nan_color, :transparent),
             :lowclip => @something(scale.props.aesprops.lowclip, Makie.automatic),
             :highclip => @something(scale.props.aesprops.highclip, Makie.automatic),

src/guides/colorbar.jl

@@ -55,7 +55,7 @@ end
 
 function continuous_colorbar(colorscale::ContinuousScale)
     label = getlabel(colorscale)
-    limits = @something colorscale.props.aesprops.colorrange colorscale.extrema
+    limits = nonsingular_colorrange(colorscale)
     is_highclipped = limits[2] < colorscale.extrema[2]
     is_lowclipped = limits[1] > colorscale.extrema[1]
 

src/guides/legend.jl

@@ -300,7 +300,7 @@ end
 datavalues_plotvalues_datalabels(aes, scale::CategoricalScale) = datavalues(scale), plotvalues(scale), datalabels(scale)
 function datavalues_plotvalues_datalabels(aes::Type{AesMarkerSize}, scale::ContinuousScale)
     props = scale.props.aesprops::AesMarkerSizeContinuousProps
-    _, s_extrema = strip_units(scale, collect(scale.extrema))
+    _, s_extrema = strip_units(scale, collect(nonsingular_limits(scale.extrema)))
     tickvalues, ticklabels = Makie.get_ticks(props.ticks, identity, props.tickformat, s_extrema...)
     t_extrema = extrema(tickvalues)
     if t_extrema[1] < s_extrema[1] || t_extrema[2] > s_extrema[2]

test/reference_tests.jl

@@ -1331,4 +1331,21 @@ reftest("hidden axis labels col row") do
         mapping(:x, :y, row = :group)
     draw!(f[1, 2], rowspec, facet = (; linkxaxes = false))
     f
-end
+end
+
+reftest("singular color and markersize limits", true) do
+    f = Figure(size = (500, 600))
+    fg = draw!(f[1, 1], pregrouped([1:3], [1:2], [ones(3, 2)]) * visual(Heatmap))
+    colorbar!(f[1, 2], fg)
+    fg2 = draw!(f[2, 1], pregrouped([1:3], [1:2], [zeros(3, 2)]) * visual(Heatmap))
+    colorbar!(f[2, 2], fg2)
+    fg3 = draw!(f[3, 1], pregrouped([1:3], [1:2], [-2 .* ones(3, 2)]) * visual(Heatmap))
+    colorbar!(f[3, 2], fg3)
+    fg4 = draw!(f[1, 3], mapping(1:5, 1:5, markersize = ones(5)) * visual(Scatter))
+    legend!(f[1, 4], fg4)
+    fg5 = draw!(f[2, 3], mapping(1:5, 1:5, markersize = zeros(5)) * visual(Scatter))
+    legend!(f[2, 4], fg5)
+    fg6 = draw!(f[3, 3], mapping(1:5, 1:5, markersize = -2 .* ones(5)) * visual(Scatter))
+    legend!(f[3, 4], fg6)
+    f
+end