title: Using xlrd, how to replace missing/NaN cells
body: <p>I am using python to plot data from excel files. I am having problems with  cells which have missing data and are filled with 'NA'.
  I have worked out how to replace them with 0, but can I replace them with blank cells instead? (Currently they mess up my graphs with the data going to 0 all over the place)
  Here is my script:</p>
  
  <pre><code>import xlrd
  
  book= xlrd.open_workbook('CHATS_tracegases_allLevels.xls')
  level_1=book.sheet_by_index(0)
  
  # Level 1 variables
  time_1=level_1.col_values(0, start_rowx=10, end_rowx=None)
  ozone_1=level_1.col_values(2, start_rowx=10, end_rowx=None)
  
  ozone_1 = [float(i) if i != 'NA' else 0 for i in ozone_1]
  plot(time_1, ozone_1)
  show()
  </code></pre>
  
  <p>Also any advice on how I can write my script better would be much appreciated (I have 6 sheets, with 8 variables in each sheet which I need to plot. So if I write it all out like the example above, it gets super huge and probably very inefficient)</p>
  
  <p>Thanks!</p>