title: Plotting error "memory exhausted or requested size too large"
body: <p>If I want to draw a <code>sin(x)</code> graph using octave, I would do </p>
  
  <pre><code>x = -6:0.1:6;
  plot (x, sin(x));
  </code></pre>
  
  <p>and this works.</p>
  
  <p>I wanted to draw a sigmoid function so I tried</p>
  
  <pre><code>x = -6:0.1:6;
  plot (x, 1/(1+exp(-x)));
  </code></pre>
  
  <p>But this gives me </p>
  
  <pre><code>error: memory exhausted or requested size too large for range of Octave's index type -- trying to return to prompt
  </code></pre>
  
  <p>I tried with <code>x = -4:0.2:4;</code> and this time got  </p>
  
  <pre><code>error: invalid conversion of NDArray to Matrix
  error: evaluating argument list element number 2
  </code></pre>
  
  <p>What is the problem?</p>