Bug fix to line drawing. Added feature setPointsPerWindow(int nPoints).

* - Bug fix: Changed line drawing from line(x, y1, x, y2) to line(x1,
y1, x2, y2) to
*  avoid problem in Processing 2.0b8 that draws nothing if calling
line(x, y, x, y)
*
* - New Feature: Added public void setPointsPerWindow(int nPoints) so
that the amount
*   (time window) of data displayed on the screen can be easily changed.