I started out with flot after noticing the high cost of Highcharts. But now have been switching out flot with dimple.js and d3.js. The charts are much better with dimple/d3.
Interesting. I considered graph libs based on-top of D3. But D3 is equally big in JS file size to JQuery. And SVG wasn't supported in older Android (2.3) and slower in Firefox and Chrome 3 years ago. So flot with its Canvas rendering "won" in the end.