Plotting/Charting tools for a LARGE set of points (25k+)

I've been looking around the web for a plotting tool to use w/ my RoR
app. I've tried already amCharts and PlotKit, and I know about the
existence of XML/SWF, FusionCharts, Gruff, Scruffy, ChartDirector
(never tested or tried though). The main problem is to find a good
harmony between good user interaction features (ajax, zoom
capabilities, many selectable data series in a plot etc) and the
ability to deal with a large set of points (25k+). amCharts would be
almost perfect, but w/ more than 3k points it gets quite slow. I
wanted some tool that when the graph is totally zoomed out, showed
only a few points, enough to get an idea, and showed the other points
only when it was zoomed in enough. Any clues? I would be willing to go
"the proprietary way" if there is a good solution there... =) thanks a
lot in advance.