title: How to hold on pylab.semilogy figures?
body: <p>How to hold on two figures of semilogy graphics of pylab? I've tried with pylab.hold(True) but it open two different windows, for two calls to my graphics function (i need the graphics in just one window). Here the code:  </p>
  
  <p>edit1 - here's more complete code, and the correction of name uses:</p>
  
  <pre><code>(import pylab as pl)
  </code></pre>
  
  <p>edit 2: i need to hold on the grafics of two consecutive calls to graphics function:
  call to graphics in the module:</p>
  
  <pre><code>graphics(pet, SNR_dB)
  graphics(another_pet, SNR_dB)
  </code></pre>
  
  <p>whit a pl.array and a list (with the same dimention), for example:
  (Generated whit arange(0,18))</p>
  
  <pre><code>SNR_dB:  [ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17]
  </code></pre>
  
  <p>And the Pe, generated with another calcules in the script:</p>
  
  <pre><code>Pe:  [0.29576000000000002, 0.24626000000000001, 0.19703999999999999, 0.15298, 0.11018, 0.074139999999999998, 0.04616, 0.024400000000000002, 0.01204, 0.0050000000000000001, 0.0018600000000000001, 0.00040000000000000002, 4.0000000000000003e-05, 0.0, 0.0, 0.0, 0.0, 0.0] 
  [0.16062000000000001, 0.13156000000000001, 0.10384, 0.079699999999999993, 0.056710000000000003, 0.037859999999999998, 0.023400000000000001, 0.01222, 0.0060400000000000002, 0.0025000000000000001, 0.00093000000000000005, 0.00020000000000000001, 2.0000000000000002e-05, 0.0, 0.0, 0.0, 0.0, 0.0]
  </code></pre>
  
  <p>The graphics function is:</p>
  
  <pre><code>def graphics(Pe, SNR_dB):
  
      pl.semilogy(SNR_dB, Pe[0],linewidth=1)
      pl.hold(True)
      pl.semilogy(SNR_dB, Pe[1],linewidth=2)
      pl.legend(('Symbol', 'bit'))
  
      pl.xlim([0, max(SNR_dB)])
      pl.ylim([1e-10,1])
  
      pl.title('Error vs SNR_dB')
      pl.grid(True)
      pl.xlabel('SNR_dB')
      pl.ylabel('Pe')
      pl.show()
  </code></pre>
  
  <p>This two calls to "graphics" generates two figures, one for each call. I need that in just one figure. How i do?</p>