title: How to make nonsymbolic plot_vector_field in sage?
body: <p>I have a function f(x,y) whose outcome is random (I take mean from 20 random numbers depending on x and y). I see no way to modify this function to make it symbolic. </p>
  
  <p>And when I run</p>
  
  <pre><code>    x,y = var('x,y') 
      d = plot_vector_field((f(x),x), (x,0,1), (y,0,1))
  </code></pre>
  
  <p>it says it can't cast symbolic expression to real or rationa number. In fact it stops when I write:</p>
  
  <pre><code>    a=matrix(RR,1,N)
      a[0]=x
  </code></pre>
  
  <p>What is the way to change this variable to real numbers in the beginning, compute f(x) and draw a vector field? Or just draw a lot of arrows with slope (f(x),x)?</p>