cache initial autosize vals -> use them on relayout

- so that relayout(gd, 'width', null) reset the
  graph to its initial width

Author: etpinard

src/plot_api/plot_api.js

@@ -1821,10 +1821,12 @@ function _relayout(gd, aobj) {
         // op and has no flag.
         undoit[ai] = (pleaf === 'reverse') ? vi : p.get();
 
-        // tweak width / height update value when null such that
-        // new dimensions are honored in Plots.supplyDefaults
+        // Setting width or height to null must reset the graph's width / height
+        // back to its initial value as computed during the first pass in Plots.plotAutoSize.
+        //
+        // To do so, we must manually set them back here using the _initialAutoSize cache.
         if(['width', 'height'].indexOf(ai) !== -1 && vi === null) {
-            vi = 'initial';
+            gd._fullLayout[ai] = gd._initialAutoSize[ai];
         }
         // check autorange vs range
         else if(pleafPlus.match(/^[xyz]axis[0-9]*\.range(\[[0|1]\])?$/)) {

src/plots/plots.js

@@ -998,6 +998,12 @@ plots.plotAutoSize = function plotAutoSize(gd, layout, fullLayout) {
         if(heightHasChanged) fullLayout.height = newHeight;
     }
 
+    // cache initial autosize value, used in relayout when
+    // width or height values are set to null
+    if(!gd._initialAutoSize) {
+        gd._initialAutoSize = { width: newWidth, height: newHeight };
+    }
+
     plots.sanitizeMargins(fullLayout);
 };
 
@@ -1117,6 +1123,7 @@ plots.purge = function(gd) {
     delete gd._lastHoverTime;
     delete gd._transitionData;
     delete gd._transitioning;
+    delete gd._initialAutoSize;
 
     // remove all event listeners
     if(gd.removeAllListeners) gd.removeAllListeners();