I find it hard to focus on specific parts of a chart once it contains lots of data points: the navigator's "handles" can be so close to each other that it is unusable. My suggestion is the following: show a 2nd navigator if there's a "more-specific data" to show (meaning - the narrowed area itself contains many points).

To demonstrate my suggestion, I implemented this with two separate charts, one sitting on top of the other, with a slightly shorter height (so that the original's chart navigator is still visible):
http://jsfiddle.net/QNV66/

Note that this example has only 100 points. It is much clearer to understand the need when viewing a chart with thousands of points.
Also, I also enabled here a "toggle" button for the zooming chart.