add mathjax subtitle mock; add addl padding bw sutbtitle and title when title is mathjax

Author: emilykl

src/components/titles/index.js

@@ -200,7 +200,9 @@ function draw(gd, titleClass, options) {
             if(titleElMathGroup && subtitleEl) {
                 var titleMathHeight = titleElMathGroup.node().getBBox().height;
                 if(titleMathHeight) {
-                    var newSubtitleY = Number(subtitleEl.attr('y')) + titleMathHeight;
+                    // Increase the y position of the subtitle by the height of the title,
+                    // plus a (somewhat arbitrary) 10px of padding
+                    var newSubtitleY = Number(subtitleEl.attr('y')) + titleMathHeight + 10;
                     subtitleEl.attr('y', newSubtitleY);
                 }
             }

test/image/mocks/zz-subtitle-mathjax.json

@@ -0,0 +1,17 @@
+{
+    "data": [
+        {
+            "x": ["a", "b", "c", "d"],
+            "y": [100, 23, 27, 40],
+            "type": "bar"
+        }
+    ],
+    "layout": {
+        "title": {
+            "text": "$E=mc^2$",
+            "subtitle": {
+                "text": "$y = ax^2 + bx + c$"
+            }
+        }
+    }
+}