title: Plot eigenvalues of matrix as function of matrix element
body: <p>I have never used python before, but need it now because I'm working on a colleague's project. I want to plot the eigenvalues of a matrix as a function of an element of the matrix, i.e. my matrix looks like</p>
  
  <pre><code>    M=[[40,0,4],[0,0,4],[4,4,x]]
  </code></pre>
  
  <p>where x is a variable. So the matrix has three eigenvalues</p>
  
  <pre><code>    eig=numpy.linalg.eigvals(M)
      eig1=eig[0]
      eig2=eig[1]
      eig3=eig[0]
  </code></pre>
  
  <p>But how can I plot this as a function of x? Thanks for your help in advance</p>