float column names leads to error in boxplot -> index.py [+ fix suggestion]
I am using Python 3, it might be a source of the problems.
I have a dataframe with columns ['genotype_cont', 0, 2.5, 5]. Trying to call attribute `boxplot` I am getting following problem:
# Code position:  index.py, line 1344:

Original line:
        theDiff = sorted(set(self) - set(other))

fix:

```
    def keyfunction(x):
        if isinstance(x, int): 
            v = '0%d' % x
        else: 
            v = x[0]
        return v

    theDiff = sorted(set(self) - set(other), key=keyfunction)
```
