Error in line 36 of p.py

Hello Carlos,

        I found the results of a pchart were not the same as the ones given in tools like Excel or Minitab.
Taking a deeper look into the code I found something that I believe should be a little bit different.
```

data2 = data / sizes
#pbar = np.mean(data2)
pbar = np.mean(data)/np.mean(sizes)
```
Whit this little change the numbers matched Minitab results.