make init_notebook_mode idempotent

Fixes plotly#458

Running `init_notebook_mode` injects plotly.js into the output cell.
Before, if you ran it twice, it would clear the output cell on the
second call and remove plotly.js from DOM.
This was deceiving, because any existing plots on the page wouldn’t
disappear even though their underlying plotly.js was no longer present.
But, if you refresh the page, the plots would not render (as plotly.js
was no longer in the output cell)

Author: chriddyp

plotly/offline/offline.py

@@ -60,15 +60,15 @@ def init_notebook_mode():
     from IPython.display import HTML, display
 
     global __PLOTLY_OFFLINE_INITIALIZED
-    if not __PLOTLY_OFFLINE_INITIALIZED:
-        display(HTML("<script type='text/javascript'>" +
-                     "define('plotly', function(require, exports, module) {" +
-                     get_plotlyjs() +
-                     "});" +
-                     "require(['plotly'], function(Plotly) {" +
-                     "window.Plotly = Plotly;" +
-                     "});" +
-                     "</script>"))
+    # ALways display the script so that init_notebook_mode is idempotent
+    display(HTML("<script type='text/javascript'>" +
+                 "define('plotly', function(require, exports, module) {" +
+                 get_plotlyjs() +
+                 "});" +
+                 "require(['plotly'], function(Plotly) {" +
+                 "window.Plotly = Plotly;" +
+                 "});" +
+                 "</script>"))
     __PLOTLY_OFFLINE_INITIALIZED = True