diff --git a/CHANGELOG.md b/CHANGELOG.md
index 95e2e9a..0913b8c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,22 @@ because a changelog assembled after the fact loses exactly the reasoning that ma
 keeping. The `[Unreleased]` target below is provisional — confirm it when the first entry is
 written.
 
+## [Unreleased] — targeting 0.2.2
+
+### New Features
+
+- **`spectrumplot` draws a reduced basis's spectrum, and `Makie` joins `[weakdeps]`, at `"0.24"`.**
+  `spectrumplot(σ::AbstractVector{<:Real}; title = "", ylabel = "", size = (620, 380))` draws `σ`
+  against its index on a log10 axis and returns a `Makie.FigureAxisPlot`. It lives in the extension
+  `ReducedBasisMethodsMakieExt`, which loads with `Makie`, so without `Makie` the call is a
+  `MethodError`; `save` needs a backend such as `CairoMakie`.
+
+### Changed
+
+- **The docs environment drops `Weave` and `Plots`, and `get_DEIM_interpolation_matrix` carries a
+  `jldoctest`.** No page used either package. This is the only `jldoctest` in the package;
+  `test/quality/doctests.jl` runs it in the `doctests` group.
+
 ## [0.2.1] — 2026-10-14
 
 ### New Features
diff --git a/Project.toml b/Project.toml
index a09871b..1fdd0c3 100644
--- a/Project.toml
+++ b/Project.toml
@@ -11,10 +11,17 @@ MultiIndexArrays = "2f7e0951-2673-41f4-a6a8-e6772a4b2a08"
 ReducedComplexityModeling = "abb4722c-1d8e-48eb-a3bd-45b3116bfaba"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 
+[weakdeps]
+Makie = "ee78f7c6-11fb-53f2-987a-cfe4a2b5a57a"
+
+[extensions]
+ReducedBasisMethodsMakieExt = "Makie"
+
 [compat]
 GeometricEquations = "0.21"
 HDF5 = "0.16, 0.17"
 LinearAlgebra = "1"
+Makie = "0.24"
 MultiIndexArrays = "0.1.2"
 ReducedComplexityModeling = "0.1.1"
 Statistics = "1"
diff --git a/docs/Project.toml b/docs/Project.toml
index 14d3885..a73d4fd 100644
--- a/docs/Project.toml
+++ b/docs/Project.toml
@@ -1,10 +1,6 @@
 [deps]
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
-Plots = "91a5bcdd-55d7-5caf-9e0b-520d859cae80"
 ReducedBasisMethods = "91da5cd7-58ae-4d67-87db-f03d1c8da121"
-Weave = "44d3d7a6-8a23-5bf8-98c5-b353f8df5ec9"
 
 [compat]
 Documenter = "1.19.0"
-Plots = "1.41.7"
-Weave = "0.10.12"
diff --git a/docs/make.jl b/docs/make.jl
index 5a7a20f..613b552 100644
--- a/docs/make.jl
+++ b/docs/make.jl
@@ -1,10 +1,5 @@
 using ReducedBasisMethods
 using Documenter
-using Weave
-
-# weave("src/poisson.jmd",
-#          out_path = "src",
-#          doctype = "github")
 
 makedocs(;
     modules = [ReducedBasisMethods],
diff --git a/ext/ReducedBasisMethodsMakieExt.jl b/ext/ReducedBasisMethodsMakieExt.jl
new file mode 100644
index 0000000..de351ad
--- /dev/null
+++ b/ext/ReducedBasisMethodsMakieExt.jl
@@ -0,0 +1,19 @@
+module ReducedBasisMethodsMakieExt
+
+using Makie
+using ReducedBasisMethods
+
+function ReducedBasisMethods.spectrumplot(σ::AbstractVector{<:Real};
+        title = "", ylabel = "", size = (620, 380))
+    all(isfinite, σ) || throw(ArgumentError("the spectrum holds a NaN or an Inf"))
+    any(>(0), σ) || throw(ArgumentError("the spectrum holds no positive value"))
+
+    # A log-scaled axis cannot draw the exact zeros of a rank-deficient snapshot set or the
+    # round-off negatives of a Gram spectrum, so every value below the floor is lifted onto it.
+    y = max.(Float64.(σ), eps(Float64) * Float64(maximum(σ)))
+
+    return lines(y; figure = (; size),
+        axis = (; title, titlealign = :left, xlabel = "i", ylabel, yscale = log10))
+end
+
+end
diff --git a/src/ReducedBasisMethods.jl b/src/ReducedBasisMethods.jl
index d69e17a..c007efa 100644
--- a/src/ReducedBasisMethods.jl
+++ b/src/ReducedBasisMethods.jl
@@ -39,6 +39,10 @@ include("reduced_equations.jl")
 
 export reduced_ode, reduced_pode
 
+include("diagnostics.jl")
+
+export spectrumplot
+
 export h5save, h5load
 
 include("reduced_tensor.jl")
diff --git a/src/algorithms/deim.jl b/src/algorithms/deim.jl
index 89c6ef1..d72753d 100644
--- a/src/algorithms/deim.jl
+++ b/src/algorithms/deim.jl
@@ -66,6 +66,15 @@ Returns the `N × k` matrix `Π` whose column `i` is the unit vector of the row
 Its element type is `float(eltype(Ψ))`, so a `Float32` basis gives a `Float32` interpolation
 matrix. The selection, and with it the `ArgumentError` for `k > N` and for a column whose
 largest residual magnitude is not finite and positive, is [`get_DEIM_indices`](@ref)'s.
+
+```jldoctest
+julia> using ReducedBasisMethods
+
+julia> get_DEIM_interpolation_matrix([1 1; 2 -1])
+2×2 Matrix{Float64}:
+ 0.0  1.0
+ 1.0  0.0
+```
 """
 function get_DEIM_interpolation_matrix(Ψ::AbstractMatrix)
     N, k = size(Ψ)
diff --git a/src/diagnostics.jl b/src/diagnostics.jl
new file mode 100644
index 0000000..10052be
--- /dev/null
+++ b/src/diagnostics.jl
@@ -0,0 +1,20 @@
+"""
+    spectrumplot(σ::AbstractVector{<:Real}; title = "", ylabel = "", size = (620, 380))
+
+Draw the spectrum `σ` of a reduced basis against its index on a log-scaled axis, in `Float64`.
+The values are drawn as given: singular values, or the squared singular values (Gram eigenvalues)
+that a `ReducedBasis` stores in `Λ`. `ylabel` names which, and `size` is the figure size.
+
+Every value below `eps(Float64) * maximum(σ)` is drawn at that floor, so the exact zeros and the
+round-off negatives of a Gram spectrum stay on the axis. A spectrum that holds a `NaN` or an `Inf`,
+or no positive value, throws an `ArgumentError`.
+
+Returns a `Makie.FigureAxisPlot`. Defined in a package extension that loads with `Makie`; `save`
+needs a backend such as `CairoMakie`.
+
+```julia
+using ReducedBasisMethods, CairoMakie
+save("spectrum.png", spectrumplot(b.Λ; ylabel = "λᵢ"))
+```
+"""
+function spectrumplot end
diff --git a/test/Project.toml b/test/Project.toml
index 7226238..45fc48b 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -1,5 +1,7 @@
 [deps]
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
+CairoMakie = "13f3f980-e62b-5c42-98c6-ff1f3baf88f0"
+Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
 GeometricBrackets = "2a9ffa85-4e0d-473f-b05e-278d80d8d306"
 GeometricIntegrators = "dcce2d33-59f6-5b8d-9047-0defad88ae06"
@@ -17,6 +19,8 @@ ReducedBasisMethods = {path = ".."}
 
 [compat]
 Aqua = "0.8.18"
+CairoMakie = "0.15"
+Documenter = "1.19.0"
 ForwardDiff = "1.4.6"
 GeometricBrackets = "0.1.1, 0.2"
 GeometricIntegrators = "0.18.5"
diff --git a/test/diagnostics.jl b/test/diagnostics.jl
new file mode 100644
index 0000000..aebbb43
--- /dev/null
+++ b/test/diagnostics.jl
@@ -0,0 +1,48 @@
+using CairoMakie
+using ReducedBasisMethods
+using Test
+
+"The plotted values of a `FigureAxisPlot`."
+plotted_y(fap) = [q[2] for q in fap.plot.positions[]]
+
+@testset "Diagnostics" begin
+    @test Base.get_extension(ReducedBasisMethods, :ReducedBasisMethodsMakieExt) isa Module
+
+    @testset "a spectrum is drawn as given and saved headless ($T)" for T in (Float64, Float32)
+        σ = T.(10.0 .^ (-(1:10)))
+        fap = spectrumplot(σ)
+        @test fap isa CairoMakie.Makie.FigureAxisPlot
+        @test eltype(eltype(fap.plot.positions[])) == Float64
+        @test plotted_y(fap) == σ
+
+        path = joinpath(mktempdir(), "spectrum.png")
+        save(path, fap)
+        @test read(path)[1:4] == UInt8[0x89, 0x50, 0x4e, 0x47]
+    end
+
+    @testset "the axis label and the figure size" begin
+        @test spectrumplot([1.0]).axis.ylabel[] == ""
+        fap = spectrumplot([1.0, 0.5]; ylabel = "λᵢ", size = (300, 200))
+        @test fap.axis.ylabel[] == "λᵢ"
+        @test Base.size(fap.figure.scene) == (300, 200)
+    end
+
+    @testset "a one-element and a BigFloat spectrum" begin
+        @test plotted_y(spectrumplot([3.0])) == [3.0]
+        @test plotted_y(spectrumplot(big.([1.0, 0.5, 0.25]))) == [1.0, 0.5, 0.25]
+    end
+
+    @testset "values below the floor are drawn at the floor ($T)" for T in (Float64, Float32)
+        # the floor is eps(Float64) * maximum(σ) = eps(Float64)
+        σ = T[1, 1e-3, 1e-20, 0, -1e-18]
+        @test plotted_y(spectrumplot(σ)) == [1, Float64(T(1e-3)), eps(), eps(), eps()]
+    end
+
+    @testset "a spectrum with a NaN, an Inf or no positive value is refused" begin
+        @test_throws ArgumentError spectrumplot(Float64[])
+        @test_throws ArgumentError spectrumplot([1.0, NaN])
+        @test_throws ArgumentError spectrumplot([1.0, Inf])
+        @test_throws ArgumentError spectrumplot(zeros(3))
+        @test_throws ArgumentError spectrumplot([0.0, -1e-17])
+    end
+end
diff --git a/test/integration/load_without_makie.jl b/test/integration/load_without_makie.jl
new file mode 100644
index 0000000..75f1e3a
--- /dev/null
+++ b/test/integration/load_without_makie.jl
@@ -0,0 +1,13 @@
+using ReducedBasisMethods
+using Test
+
+# `Makie` is a weak dependency: loading the package, in an environment that has a Makie backend
+# installed, loads no Makie module, and `spectrumplot` has no method.
+@testset "Load without Makie" begin
+    code = """
+    using ReducedBasisMethods, Test
+    @test !any(m -> occursin("Makie", m.name), keys(Base.loaded_modules))
+    @test_throws MethodError spectrumplot([1.0, 0.5])
+    """
+    @test success(`$(Base.julia_cmd()) --startup-file=no --project=$(Base.active_project()) -e $code`)
+end
diff --git a/test/quality/docs.jl b/test/quality/docs.jl
new file mode 100644
index 0000000..cbe58ff
--- /dev/null
+++ b/test/quality/docs.jl
@@ -0,0 +1,12 @@
+using ReducedBasisMethods
+using Test
+
+@testset "no physical word in the code or the documentation" begin
+    # The package knows no model. `CHANGELOG.md` and `reference/` may name one.
+    root = pkgdir(ReducedBasisMethods)
+    files = [joinpath(path, file) for dir in ("src", "ext", "docs/src")
+             for (path, _, names) in walkdir(joinpath(root, dir)) for file in names]
+    for file in [files; joinpath(root, "docs", "make.jl")]
+        @test !occursin(r"vlasov|particle"i, read(file, String))
+    end
+end
diff --git a/test/quality/doctests.jl b/test/quality/doctests.jl
new file mode 100644
index 0000000..ecb34f5
--- /dev/null
+++ b/test/quality/doctests.jl
@@ -0,0 +1,5 @@
+using ReducedBasisMethods
+using Documenter: doctest
+
+# The docstring doctests. `manual = false` leaves the manual's doctests to `docs/make.jl`.
+doctest(ReducedBasisMethods; manual = false)
diff --git a/test/runtests.jl b/test/runtests.jl
index fdbc3f8..3f03610 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -4,6 +4,7 @@ const GROUPS = isempty(ARGS) ? ["core", "slow"] : ARGS
 
 if "core" in GROUPS
     @safetestset "Aqua" include("quality/aqua.jl")
+    @safetestset "Docs" include("quality/docs.jl")
     @safetestset "Generic-only skeleton" include("integration/skeleton.jl")
     @safetestset "Reference spectra script" include("integration/reference_spectra.jl")
     @safetestset "Reduced basis" include("reducedbasis.jl")
@@ -16,3 +17,10 @@ if "core" in GROUPS
     @safetestset "Symplectic DEIM" include("algorithms/symplectic_deim.jl")
     @safetestset "Reduced equations" include("reduced_equations.jl")
 end
+if "slow" in GROUPS
+    @safetestset "Load without Makie" include("integration/load_without_makie.jl")
+    @safetestset "Diagnostics" include("diagnostics.jl")
+end
+if "doctests" in GROUPS
+    @safetestset "Doctests" include("quality/doctests.jl")
+end