title: Secondary y-axis in sympy
body: <p>Sympy plotting has recently evolved a lot, but I cannot find a way to plot a second expression on a secondary y-axis. Below, I show an example where I combine two plots, but would like the second to use a secondary axis:</p>
  <pre><code>&gt;&gt;&gt; from sympy import symbols
  &gt;&gt;&gt; from sympy.plotting import plot
  &gt;&gt;&gt; x = symbols('x')
  &gt;&gt;&gt; p1 = plot(x**4, (x,10,20), label='$x^4$', show=False, legend=True)
  &gt;&gt;&gt; p2 = plot(x, (x,10,20), line_color='red', label='$x$', show=False, legend= True)
  &gt;&gt;&gt; p1.extend(p2)
  &gt;&gt;&gt; p1.show()
  </code></pre>
  <p>Incidentally, is there a way to also modify the line style, e.g. make one of them dashed? The documentation suggests to use the <code>_backend</code> module for fine tuning, but it is not clear to me how to achieve this. See e.g. <a href="https://stackoverflow.com/questions/43869393/how-do-i-use-the-backend-attribute-of-a-sympy-plot">How do I use the `_backend` attribute of a Sympy `plot`</a>.</p>