I think flot is a great jquery-based tool, so I always like to point it out when someone is using it well. In doing so, I was recently asked, "How is [flot] different than Excel and what are its advantages?"
If you want to embed a chart in a webpage, you can create it in Excel, export to an image and then put that image onto your page. This process works, but it is more static than flot on two levels.
First, if you want to add new data to your dataset and have it show up in your chart on your site, you would have to repeat the entire process that you went through with Excel. Create, upload, embed. With flot, since the data and the instructions for how to plot are both passed through with each load of a webpage, you just have to update your dataset and don't have to touch the plot.
Second, since flot tells your browser how to display data, you can make your plot interactive very easily. For example, you can make it so that you can hover over a point and its coordinates will appear. An image loaded into the webpage, however, wouldn't "know" when your mouse was over a point. The image is just a bunch of pixels colored in a certain order.
If you'd like to see flot in action, here's a link to the author's examples.