Pie chart 

Here is initial pie chart implementation.

It works well except for changing pie chart <--> other chart raise some error, like
```
Error: Invalid value for <g> attribute transform="translate(NaN,NaN)" d3.js:596
Error: Invalid value for <path> attribute d="M0,-84A84,84 0 1,1 NaN,NaNL0,0Z" 
```

In progress ...