Fix Plotly.plot promise chain and test with mathjax image

Author: alexcjohnson

src/plot_api/plot_api.js

@@ -107,7 +107,7 @@ Plotly.plot = function(gd, data, layout, config) {
     // any part of the plotting code can push to gd._promises, then
     // before we move to the next step, we check that they're all
     // complete, and empty out the promise list again.
-    gd._promises = [];
+    if(!Array.isArray(gd._promises)) gd._promises = [];
 
     var graphWasEmpty = ((gd.data || []).length === 0 && Array.isArray(data));
 
@@ -384,14 +384,16 @@ Plotly.plot = function(gd, data, layout, config) {
         drawData,
         finalDraw,
         initInteractions,
-        Plots.rehover
+        Plots.rehover,
+        Plots.previousPromises
     ];
 
-    Lib.syncOrAsync(seq, gd);
-
     // even if everything we did was synchronous, return a promise
     // so that the caller doesn't care which route we took
-    return Promise.all(gd._promises).then(function() {
+    var plotDone = Lib.syncOrAsync(seq, gd);
+    if(!plotDone || !plotDone.then) plotDone = Promise.resolve();
+
+    return plotDone.then(function() {
         gd.emit('plotly_afterplot');
         return gd;
     });