Update Part 1. Data Wrangling with Pandas.ipynb

this line (Missing data) section missing data, returns a "name 'nan' is not defined"

 data.ix[7, 'year'] = nan

Cast to float so that the cell compiles correctly. 

 data.ix[7, 'year'] = float('Nan')

Alternative is to use the numpy nan

 data.ix[7, 'year'] = np.nan
Merge pull request #1 from lluang/patch-1

Update Part 1. Data Wrangling with Pandas.ipynb data.ix[7, 'year'] = nan   "name 'nan' is not defined"