title: nan in interp1d scipy
body: <p>I have the following code that I am working on in python with interp1d and it seems that the output of the interp1d times the query points outputs the beginning values of array as <code>NaN</code>. Why? </p>
  
  <pre><code>Freq_Vector = np.arange(0,22051,1)
  Freq_ref = np.array([20,25,31.5,40,50,63,80,100,125,160,200,250,315,400,500,630,750,800,1000,1250,1500,1600,2000,2500,3000,3150,4000,5000,6000,6300,8000,9000,10000,11200,12500,14000,15000,16000,18000,20000])   
  W_ref=-1*np.array([39.6,32,25.85,21.4,18.5,15.9,14.1,12.4,11,9.6,8.3,7.4,6.2,4.8,3.8,3.3,2.9,2.6,2.6,4.5,5.4,6.1,8.5,10.4,7.3,7,6.6,7,9.2,10.2,12.2,10.8,10.1,12.7,15,18.2,23.8,32.3,45.5,50])
  if FreqVector[-1] &gt; Freq_ref[-1]:
      Freq_ref[-1] = FreqVector[-1]
  WdB = interpolate.interp1d(Freq_ref,W_ref,kind='cubic',axis=-1, copy=True, bounds_error=False, fill_value=np.nan)(FreqVector)
  </code></pre>
  
  <p>The first 20 values in WdB are : </p>
  
  <pre><code>00000 = {float64} nan
  00001 = {float64} nan
  00002 = {float64} nan
  00003 = {float64} nan
  00004 = {float64} nan
  00005 = {float64} nan
  00006 = {float64} nan
  00007 = {float64} nan
  00008 = {float64} nan
  00009 = {float64} nan
  00010 = {float64} nan
  00011 = {float64} nan
  00012 = {float64} nan
  00013 = {float64} nan
  00014 = {float64} nan
  00015 = {float64} nan
  00016 = {float64} nan
  00017 = {float64} nan
  00018 = {float64} nan
  00019 = {float64} nan
  00020 = {float64} -39.6
  00021 = {float64} -37.826313148
  </code></pre>
  
  <p>The following is the same outputted in maltab for the first 20 values:</p>
  
  <pre><code>-58.0424562952059
  -59.2576965087483
  -60.1150845850336
  -60.6367649499501
  -60.8448820293863
  -60.7615802492306
  -60.4090040353715
  -59.8092978136973
  -58.9846060100965
  -57.9570730504576
  -56.7488433606689
  -55.3820613666188
  -53.8788714941959
  -52.2614181692886
  -50.5518458177851
  -48.7722988655741
  -46.9449217385440
  -45.0918588625830
  -43.2352546635798
  -41.3972535674226
  -39.6000000000000
  -37.8656383872004    
  </code></pre>
  
  <p>How can I avoid this and actually have real values like matlab does with interp1d? </p>