The feature you want is requested here in the Flot Google group. It doesn't look like it was ever implemented (there's nothing in the API about putting any labels inside the chart itself). I think that the answer to your question is that No, it's not possible at this time to show values next to certain points on lines inside the graph.
Ole Larson, head developer on Flot, mentioned that showing labels inside the chart is different than anything else on FLot and that they would have to think about how to extend the API / plot parameters to do it.
That said, you might want to go post a question on the Flot forum or make a suggestion on the bug-tracker for the new feature. Ole Larson is actually really good at getting back to all the questions, bugs, and suggestions himself.