define layout.computed in the schema

Author: archmoj

src/plots/layout_attributes.js

@@ -265,6 +265,15 @@ module.exports = {
         },
         editType: 'plot'
     },
+    computed: {
+        valType: 'any',
+        role: 'info',
+        editType: 'none',
+        description: [
+            'Placeholder for exporting automargin-impacting values namely',
+            '`margin.t`, `margin.b`, `margin.l` and `margin.r` in *full-json* mode.',
+        ].join(' ')
+    },
     paper_bgcolor: {
         valType: 'color',
         role: 'style',

src/plots/plots.js

@@ -2164,7 +2164,7 @@ plots.graphJson = function(gd, dataonly, mode, output, useDefaults, includeConfi
         obj.layout = stripObj(layout);
         if(useDefaults) {
             var gs = layout._size;
-            obj.layout._computed = {
+            obj.layout.computed = {
                 margin: {
                     b: gs.b,
                     l: gs.l,

test/jasmine/tests/toimage_test.js

@@ -307,7 +307,7 @@ describe('Plotly.toImage', function() {
             Plotly.toImage(pieAutoMargin, imgOpts)
             .then(function(fig) {
                 fig = JSON.parse(fig);
-                var computed = fig.layout._computed;
+                var computed = fig.layout.computed;
                 expect(computed).toBeDefined('no computed');
                 expect(computed.margin).toBeDefined('no computed margin');
                 expect(computed.margin.t).toBeDefined('no top');