title: matplotlib - save plot lines without background and borders and transparency 8 bits alpha channel issue
body: <p>I am struggling to save the plot from matplotlib without background and borders.
  In particular I would like to export in two formats:</p>
  
  <ul>
  <li>svg, with only the plot lines (no background, no axis, no frames, no borders)</li>
  <li>png, with transparent background and the bounding box fitting exactly the plot.</li>
  </ul>
  
  <p>To be more precise, I give an example.</p>
  
  <pre><code>i = 1000
  j = 1024
  periods = 6
  x = np.array([np.linspace(0, (2 * (periods) * np.pi), i)]).T
  x = np.repeat(x, j, axis=1)
  n = (1 * (np.random.normal(size=(j))) *
       np.random.uniform(low=1, high=1, size=j))[:, np.newaxis]
  n = np.repeat(n, i, axis=1).T
  y = np.sin(x) * (np.sin(n)+4) + 0.5 * n**2
  xout = np.array([np.linspace(0, 2 * (2 * (periods) * np.pi), 2 * i)]).T
  xout = np.repeat(xout, j, axis=1)
  yout = np.concatenate((y, y[::-1]))
  f1 = plt.figure("GOOD1")
  plt.axis('off')
  plt.plot(x, y, 'b', alpha=(0.015625))
  plt.savefig("GOOD1.svg", bbox_inches='tight', transparent=True)
  plt.savefig("GOOD1.png", bbox_inches='tight', transparent=True)
  </code></pre>
  
  <p>This looks similar to what I would like to export but still has a small space on the right in both the png and svg images; the svg has also a background.</p>
  
  <p>The second question is about the alpha channel.
  I understood it is limited to 8 bits and hence decreasing the alpha value a bit much more (as to 0.005) will make the plot disappear.
  Referring to the previous code, is there a way to plot even more lines with an increased transparency without losing the plot at all?</p>