ENH: Don't omit columns with categorical data in df.hist() 
For some strange reason `df.hist`  can plot categorical features where features are given as string values. 

     df.cat.hist()

If the dataframe in column "cat" contains string type values the histogram is generated correctly. 

But when called like this `df.hist()` the  corresponding plot is left out. Along the lines, it would be nice if `df.bool.hist()`, where "bool" contains boolean values, would not throw an error but would handle it as binary category value and label it True and False. I am realizing there are many improvements possible with df.hist(). Not sure if I should open a new issue for each one. 