Closes #31; Fixed real-time line plot thickness

The plot now takes the device pixel ratio into account to ensure that lines are rendered with the same apparent thickness across displays with different pixel ratios. Also changed the range of the y scale so that partial line segments wouldn't be clipped by the edges of the canvas.
This seems to assume that `@pixelRatio / 2 ` will evenly divide; not all devices have a ratio that is an integer (for instance, the Nexus7 has a pixel device ratio of around 1.325) - is there a way to handle remainders?