Add dataset trendline segregation to /caller view

![](http://cl.ly/image/0q0k2c1f0t3K/Screen%20Shot%202014-09-03%20at%203.11.13%20PM.png)

Now the caller trends view shows separate trendlines per dataset, making this view actually useful now.

Significant code tidying up as well.

fixes #1 
how about naming this something more descriptive like `makeSvgPathString`?