set subtitle font size based on title font size

Author: emilykl

src/components/titles/index.js

@@ -247,7 +247,7 @@ function draw(gd, titleClass, options) {
             subtitleEl.style('opacity', subtitleOpacity * Color.opacity(subFontColor))
             .call(Drawing.font, {
                 color: Color.rgb(subFontColor),
-                size: d3.round(subFontSize * 0.8, 2),
+                size: d3.round(subFontSize, 2),
                 family: subFontFamily,
                 weight: subFontWeight,
                 style: subFontStyle,

src/plots/plots.js

@@ -1498,7 +1498,7 @@ plots.supplyLayoutGlobalDefaults = function(layoutIn, layoutOut, formatObj) {
     coerce('title.subtitle.text', layoutOut._dfltTitle.plotsubtitle);
     Lib.coerceFont(coerce, 'title.subtitle.font', font, {
         overrideDflt: {
-            size: Math.round(fontSize * 1.2)
+            size: Math.round(layoutOut.title.font.size * 0.65)
         }
     });