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.
Merge pull request #38 from fastly/rsandor/fix-real-time-line-thickness

Closes #31; Fixed real-time line plot thickness