Updated Enrollment Trend Styling

After far too many hours the enrollment chart has been updated to more closely resemble the design below. Note that some minor debt is introduced in the form of hard-coded heights and widths. This seems to be unavoidable as manipulating SVG is not a simple task.

Original
https://photos-4.dropbox.com/t/0/AAAnOqAg1LSrYw5wZ2bLYvrVvqhdxXXZ8gSynEi0pOC2SA/12/3716/png/1024x768/3/1410382800/0/2/Screenshot%202014-09-04%2012.33.22.png/u3kJepEEMloBRJ6AZYJv6rAc2c6FGrvJYKtFRcNPQe4

Design
https://photos-4.dropbox.com/t/0/AABqGNJGN2vr6MVtbhHsNPBzpcfbQbxSrOYlAUAMLQ3LOQ/12/3716/png/1024x768/3/1410382800/0/2/Screenshot%202014-09-04%2012.34.01.png/jX3EIhwFAMWX0rwzf2TS1QeoJsY-9H__znF6c-wYFf4

The design for review is shown below. Note that I opted to no longer force the minimum value for the Y-axis to zero. Doing so hides a lot of variability in the data.
![screen shot 2014-09-11 at 5 13 32 pm](https://cloud.githubusercontent.com/assets/910510/4242042/abaacd0e-39f9-11e4-8eed-8507f0541672.png)

Here is what the graph looks like when forced to zero.
![screen shot 2014-09-11 at 5 14 13 pm](https://cloud.githubusercontent.com/assets/910510/4242051/d80b65ac-39f9-11e4-8f4c-7194e6653bfd.png)

Finally, I propose (but have not implemented in the code for review) removing the "Enrollments" label. It's lonely and doesn't add any information to the graph.
![screen shot 2014-09-11 at 5 14 51 pm](https://cloud.githubusercontent.com/assets/910510/4242060/087009be-39fa-11e4-8c28-a97c82f33b68.png)

@marcotuts @shnayder @dsjen @rocha 
I find el is a bit generic.  What about `axisEl`?