add z_order (#4139)

* add z_order

* format

* add missing ,

* fix testss

* this time for real

* format [skip ci]

test/runtests.jl

@@ -38,6 +38,7 @@ end
 end
 
 for fn in (
+    "test_args.jl",
     "test_defaults.jl",
     "test_pipeline.jl",
     "test_axes.jl",

test/test_args.jl

@@ -0,0 +1,10 @@
+using Plots, Test
+
+@testset "Series Attributes" begin
+    pl = plot([[1, 2, 3], [2, 3, 4]], lw = 5)
+    @test hline!(deepcopy(pl), [1.75])[1].series_list[3][:label] ==
+          hline!(deepcopy(pl), [1.75], z_order = :front)[1].series_list[3][:label] ==
+          "y3"
+    @test hline!(deepcopy(pl), [1.75], z_order = :back)[1].series_list[1][:label] == "y3"
+    @test hline!(deepcopy(pl), [1.75], z_order = 2)[1].series_list[2][:label] == "y3"
+end