Highcharts is one of the best graphing library I have come across. It makes all the charts interactive with zooming, auto scaling, dynamic visibility of various lines/metrics etc.
Check it out at: http://www.highcharts.com/
Check it out at: http://www.highcharts.com/
I'm unlikely to even considerer another library until they match that feature.
flot.js has zooming/panning capabilities - http://people.iola.dk/olau/flot/examples/selection.html
This, assuming I understood what you were getting at.
Needless to say, HighCharts is slick - but flot.js has a nice price tag :)
There's also a "range selector" mode if you prefer the two-pane zooming interface: http://dygraphs.com/tests/range-selector.html