title: The same variable should be used in all univariate expressions being plotted
body: <p>I am using python 2.7.12 with SymPy 1.0, along with PyCharm Community Edition 2016.2.3.</p>
  
  <p>I have an expression that corresponds to the following code:</p>
  
  <pre><code>x, y = symbols('x y')
  f1 = 2*x
  f2 = 2*y
  F = f1 + f2
  </code></pre>
  
  <p>What should be the syntax to plot this ? I used all sorts of <code>plot(F)</code> or <code>plot3d(F)</code> , but keep getting the "ValueError: The same variable should be used in all univariate expressions being plotted."</p>