Add stem plots for sampled data systems

.zenodo.json

@@ -814,6 +814,11 @@
             "affiliation": "@JuliaComputing",
             "name": "Παναγιώτης Γεωργακόπουλος",
             "type": "Other"
+        },
+        {
+            "affiliation": "@ZiplineTeam",
+            "name": "Jonnie Diegelman",
+            "type": "Other"
         }
     ],
     "upload_type": "software"

PlotsBase/src/recipes.jl

@@ -1620,4 +1620,30 @@ end
     end
 end
 
+# -------------------------------------------------
+# Stem plots are useful for visualizing discretely sampled time series data
+@userplot Stem
+
+# If the second arg is a function, eagerly apply it so we can create the stem lines
+process_stem_args(x, f::Function) = (x, f.(x))
+process_stem_args(x, y) = (x, y)
+
+@recipe function f(s::Stem)
+    if length(s.args) != 2
+        throw(MethodError(stem, s.args))
+    end
+    x, y = process_stem_args(s.args...)
+    @series begin
+        primary := true
+        seriestype := :scatter
+        markerstrokewidth --> 0
+        x, y
+    end
+    @series begin
+        primary := false
+        seriestype := :line
+        [x x]', [zero(y) y]'
+    end
+end
+
 @specialize

PlotsBase/test/test_recipes.jl

@@ -143,6 +143,20 @@ with(:gr) do
         @test_nowarn show(devnull, scatter3d(1:2, 1:2, 1:2))
     end
 
+    @testset "stem" begin
+        n = 0:0.5:10
+
+        # Make sure array and function arguments work
+        @test stem(n, sin.(n)) isa PlotsBase.Plot
+        @test stem(n, sin) isa PlotsBase.Plot
+
+        # Should only accept two positional arguments, otherwise throw an error
+        @test_throws MethodError stem()
+        @test_throws MethodError stem(n)
+        @test_throws MethodError stem(n, n, n)
+        @test_throws MethodError stem(n, n, n, n)
+    end
+
     @testset "sticks" begin
         @test_nowarn show(devnull, sticks(1:2, marker = :circle))
     end