title: Why is the plotting-functions of package "ControlSystems" in Julia giving me a "UndefVarError: subplot not defined"?
body: <p>I use Julia 0.4.3 and I have updated all packages using Pkg.update().
  From the "ControlSystems" documentation, it is explicitly stated that plotting requires extra care in that the user is free to choose plotting back-end (I guess back-end means which plotting-package is used by "ControlSystems"). I have installed and like to use pyplot - hence I try the following code:</p>
  
  <pre><code>using ControlSystems
  Plots.pyplot()
  s = tf("s");
  G = 1/(s+1);
  stepplot(G);
  </code></pre>
  
  <p>Gives the error-message</p>
  
  <pre><code>ERROR: UndefVarError: subplot not defined
  in stepplot at C:\folder\.julia\v0.4\ControlSystems\src\plotting.jl81
  in stepplot at C:\folder\.julia\v0.4\ControlSystems\src\plotting.jl103
  </code></pre>
  
  <p>I have also tried the same code without the "Plots.pyplot()"-command.</p>