Improve graph / pivot behavior

**1. When there're only one group and one value selected, use group name instead of group.value as column name.**

before
![image](https://cloud.githubusercontent.com/assets/1540981/4801617/32921786-5e36-11e4-9a1f-fb09c0efc2ed.png)

after
![image](https://cloud.githubusercontent.com/assets/1540981/4801604/0fbb76da-5e36-11e4-8f4a-f234c432606a.png)

**2. Display text labels for line, area chart**

before
![image](https://cloud.githubusercontent.com/assets/1540981/4801635/7d380200-5e36-11e4-9d82-3b2d507a4e1f.png)

after
![image](https://cloud.githubusercontent.com/assets/1540981/4801641/96fc394a-5e36-11e4-9723-0a8cf8bc7403.png)


**3. When no keys, groups but values are selected, handle them proper way**

before
![image](https://cloud.githubusercontent.com/assets/1540981/4801690/56989d7a-5e37-11e4-80a9-6b6e38199e1b.png)

after
![image](https://cloud.githubusercontent.com/assets/1540981/4801679/337538da-5e37-11e4-8545-584220f61be6.png)

**4. After paragraph run, when data is changed and columns are changed, select some columns automatically if possible, instead of displaying "No data available"**

Ready to merge
awesome !