add mathjax subtitle and title to ternary-mathjax

Author: archmoj

test/image/baselines/mathjax3___ternary-mathjax.png

@@ -48,16 +48,13 @@
         }
       }
     },
-    "annotations": [
-      {
-        "showarrow": false,
-        "text": "Simple Ternary Plot with Markers",
-        "x": 0.5,
-        "y": 1.3,
-        "font": {
-          "size": 15
-        }
+    "width": 600,
+    "height": 600,
+    "title": {
+      "text": "Simple Ternary Plot with Markers",
+      "subtitle": {
+        "text": "$y = ax^2 + bx + c$"
       }
-    ]
+    }
   }
 }