Original Flot maintainer here.

I once had to deal with many million data points for an application. I ended up mip-mapping them client-side.

But regarding sampling, if it's a line chart, you can sample adaptively by checking whether the next point makes a meaningfully visible difference measured in pixels compared to its neighbours. When you tune it correctly, you can drop most points without the difference being noticeable.

I didn't find any else doing that at the time, and some people seemed to have trouble accepting it as a viable solution, but if you think about it, it doesn't actually make sense to plot say 1 million points in a line chart 1000 pixels wide. On average that would make 1000 points per pixel.