Line graph is truncating y-values

For line graphs, y-values that are floats (e.g., 96.25) are being truncated.

Instead, we should be displaying the original float value.
The x and y axis ranges should be configurable

Previously, firegrapher was ignoring min and max x and y values set by the app. 

We want the user to be able to set min and max x and y values. For example, it may not be appropriate to have the axis start at 0 in every case.
Merge pull request #2 from lauradhamilton/master

Line graph is truncating y-values