Right now, the crosshairs option draws a line of fixed width (usually quite thin). This looks cool... until you switch to a category-based chart.
In a chart with categories, the crosshair is just a thin line runnign through the centre of the category.
It would be very nice to have an option to highlight the entire category, or to make a crosshair line as wide as the distance between ticks (assuming arbitrary zoom).
(This can be done by setting plotBands in the tooltip formatter function, but that method seems rather hack-ish)