add more tests

test/test_plot.jl

@@ -0,0 +1,17 @@
+using Plots, Test
+
+@testset "Adding attributes" begin
+    pl = plot(rand(8,8), layout = 4)
+    pl_all = plot!(deepcopy(pl); label = ["test" for _ in 1:8])
+    pl_subplot = plot!(deepcopy(pl); label = ["test" for _ in 1:2], subplot = 2)
+    pl_series = plot!(deepcopy(pl); label = "test", series = 5)
+    for series in pl_all.series_list
+        @test series[:label] == "test"
+    end
+    @test pl_subplot[1][1][:label] == "y1"
+    for series in pl_subplot[2].series_list
+        @test series[:label] == "test"
+    end
+    @test pl_series[1][1][:label] == "y1"
+    @test pl_series.series_list[5][:label] == "test"
+end