modify x-label printouts

trim x-labels to smallest non-zero decimal, and drop decimal place if
  the x-value is an integer

data
![hist](https://cloud.githubusercontent.com/assets/529129/2726605/403b4b7a-c5be-11e3-8552-fc9f5f37e581.png)
cmd
![cmd](https://cloud.githubusercontent.com/assets/529129/2726607/46baffa4-c5be-11e3-94f8-78484e36b7d5.png)
before
![before](https://cloud.githubusercontent.com/assets/529129/2726609/54d1f782-c5be-11e3-8547-46da49e21d68.png)
after
![after](https://cloud.githubusercontent.com/assets/529129/2726611/5a578df2-c5be-11e3-81e2-cf36ad9b88e5.png)


cmd 2
![cmd 2](https://cloud.githubusercontent.com/assets/529129/2726629/e9962352-c5be-11e3-83b4-ddf92ed1086f.png)
unchanged
![unchanged](https://cloud.githubusercontent.com/assets/529129/2726631/f088cde0-c5be-11e3-8fad-080f74771a31.png)