Make layouts tolerant of having too-few items (#4247)

* Make layouts tolerant of fewer items.

* Add tests

* Remove grid kwarg

* format add `plot!` test

Co-authored-by: Simon Christ <christ@cell.uni-hannover.de>

Author: BioTurboNick

src/layouts.jl

@@ -455,9 +455,9 @@ end
 
 function layout_args(plotattributes::AKW, n_override::Integer)
     layout, n = layout_args(n_override, get(plotattributes, :layout, n_override))
-    if n != n_override
+    if n < n_override
         error(
-            "When doing layout, n ($n) != n_override ($(n_override)).  You're probably trying to force existing plots into a layout that doesn't fit them.",
+            "When doing layout, n ($n) < n_override ($(n_override)).  You're probably trying to force existing plots into a layout that doesn't fit them.",
         )
     end
     layout, n

test/test_layouts.jl

@@ -33,6 +33,26 @@ end
     @test p[3][:framestyle] === :none
 end
 
+@testset "Allowed subplot counts" begin
+    p = plot(plot(1:2); layout = grid(2, 2))
+    @test length(p) == 1
+    p = plot(plot(1:2), plot(1:2); layout = grid(2, 2))
+    @test length(p) == 2
+    p = plot(plot(1:2), plot(1:2), plot(1:2); layout = grid(2, 2))
+    @test length(p) == 3
+    @test length(plot!(p, plot(1:2))) == 4
+    p = plot(plot(1:2), plot(1:2), plot(1:2), plot(1:2); layout = grid(2, 2))
+    @test length(p) == 4
+    @test_throws ErrorException plot(
+        plot(1:2),
+        plot(1:2),
+        plot(1:2),
+        plot(1:2),
+        plot(1:2);
+        layout = grid(2, 2),
+    )
+end
+
 @testset "Coverage" begin
     p = plot((plot(i) for i in 1:4)..., layout = (2, 2))