text: Title: [BUG] - Cannot set non-string value {value} into a StringArray with klib.cat_plot
  Body: Once running `klib.cat_plot(df_cleaned)` I get this error:
  `Cannot set non-string value '2' into a StringArray.`
  
  <img width="834" alt="Screen Shot 2020-10-05 at 11 01 32 AM" src="https://user-images.githubusercontent.com/18530257/95060155-2ce92680-06fa-11eb-9532-38c051e3bcc0.png">
  
  Python version: Python 3.6.9
  Pandas version: 1.1.2
  
  My data look like this:
  0   vmid               string  
   1   subscriptionid     category
   2   deploymentid       category
   3   vmcreated          int32   
   4   vmdeleted          int32   
   5   maxcpu             float32 
   6   avgcpu             float32 
   7   p95maxcpu          float32 
   8   vmcategory         category
   9   vmcorecountbucket  category
   10  vmmemorybucket     category
   11  lifetime_h         float32 