Renamed return value from Read call from button to event... EVERYWHERE

Demo_Graph_Element.py

@@ -45,8 +45,8 @@ def main():
     while True:
         time.sleep(.2)
 
-        button, values = window.ReadNonBlocking()
-        if button == 'Quit' or values is None:
+        event, values = window.ReadNonBlocking()
+        if event == 'Quit' or values is None:
             break
         if g_response_time is None or prev_response_time == g_response_time:
             continue