So, this would probably be an odd, out-of-the-box feature for flot to have, as it would not have a desirable behavior for dense data sets. That said, it would not be difficult to create a simple plug-in to achieve your desired result. You would probably want to use the 'draw' hook, and then draw something similar to what is being drawn for the plot hover event (jquery.flot.hover.js). See the
PLUGINS.md doc and the plugin section of the
API.md doc to help you get started.
Let me know if I can help clear anything up for you. Good luck!
-Jonathan