fixup mock to use period on the x axis not y

Author: archmoj

test/image/baselines/period_positioning2.png

@@ -73,6 +73,7 @@
         },
         "xaxis2": {
             "rangeslider": {"visible": false},
+            "ticklabelmode": "period",
             "tickcolor": "black",
             "anchor": "y2",
             "domain": [
@@ -107,7 +108,6 @@
             ]
         },
         "yaxis2": {
-            "ticklabelmode": "period",
             "tickcolor": "black",
             "anchor": "x2",
             "domain": [