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 line is adding additional padding to the range of the y-scale so we don't get clipping in the display. D3's linear scale is a floating-point scale and doesn't require integer values for the domain and the range. Even if the pixel ratio is some floating point value, say `3.1415`, then the y scale will end up doing the mapping all the same without a problem.