Calculates min and max y values and uses it to adjust the y_range, without complete success.
Modifies ZoomTool invocation so that it zooms along x-axis only.
Initial y_range adjustment to better fit data accomplished.
Improves y_range bound calculation by using range.bound_data on the index_mapper.
Attaches listener to xrange _low_value which recalculates the bounding y-range so that the graph fits well every time we pan or zoom.