Fix zindex_violin_box mock

Author: Farkites

test/image/mocks/zindex_violin_box.json

@@ -1,10 +1,8 @@
 {
     "data": [
-
         {
             "type": "violin",
             "name": "violin",
-            "y": 1,
             "fillcolor": "yellow",
             "x": [
                 0.1, 0.3, 0.1, 0.9, 0.6, 0.6,
@@ -14,13 +12,11 @@
         {
             "type": "box",
             "name": "box no zindex",
-            "x": 1,
             "y": [-1, 1]
         }, 
         {
             "type": "box",
             "name": "box below",
-            "x": 2,
             "y": [-1, 1],
             "zindex": -100
         }