global name 'O' is not defined in Plot pane for sin series demo

[Sam Zhang]

Hello!

Hope this is the right place for this - I just discovered gamma today. I was clicking around the demos on the front page of the site, and this one for a series expansion for sin(x) creates an error:

http://gamma.sympy.org/input/?i=series%28sin%28x%29%2C%20x%2C%20pi/2%29

Plot:
There was an error in Gamma. For reference the last five traceback entries are: Traceback (most recent call last): 
File "/base/data/home/apps/s~sympy-gamma-hrd/26.400993473311063845/app/views.py", line 278, in eval_card
  result = g.eval_card(card_name, expression, variable, parameters)
File "/base/data/home/apps/s~sympy-gamma-hrd/26.400993473311063845/app/logic/logic.py", line 322, in eval_card
  result = card.eval(evaluator, components, parameters)
File "/base/data/home/apps/s~sympy-gamma-hrd/26.400993473311063845/app/logic/resultsets.py", line 98, in eval 
  return self.card_info['eval_method'](evaluator, components, parameters)
File "/base/data/home/apps/s~sympy-gamma-hrd/26.400993473311063845/app/logic/resultsets.py", line 562, in eval_plot 
  series = series.get_segments()
File "/base/data/home/apps/s~sympy-gamma-hrd/26.400993473311063845/sympy/sympy/plotting/plot.py", line 455, in get_segments 
  f_start = f(self.start) NameError: global name 'O' is not defined