Highcharts' zoomable graphs are a killer feature at the moment. They are not difficult to use, and I use them to show 30 days worth of data (2880 points per graph) in my monitoring apps. They work on mobile and web out of the box, too.

I'm unlikely to even considerer another library until they match that feature.

When you say zoomable charts, do you mean like the one here http://www.highcharts.com/demo/dynamic-master-detail ?

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 :)

dygraphs has had that feature for about five years :)

There's also a "range selector" mode if you prefer the two-pane zooming interface: http://dygraphs.com/tests/range-selector.html

I don't see the small window which shows the x & y co ordinates of the point you are hovering at. Is it it turned off by default?

Nope, it's in the top right of the chart. It shows values when you hover over the chart.