title: Normalise between 0 and 1 ignoring NaN
body: <p>For a list of numbers ranging from <code>x</code> to <code>y</code> that may contain <code>NaN</code>, how can I normalise between 0 and 1, ignoring the <code>NaN</code> values (they stay as <code>NaN</code>).</p>
  
  <p>Typically I would use <code>MinMaxScaler</code> (<a href="http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.MinMaxScaler.html#sklearn.preprocessing.MinMaxScaler" rel="noreferrer">ref page</a>) from <code>sklearn.preprocessing</code>, but this cannot handle <code>NaN</code> and recommends imputing the values based on mean or median etc. it doesn't offer the option to ignore all the <code>NaN</code> values.</p>