Flot Topbar Plugin
Flot Charts have a short file to guide you through creating a plugin, and this task was a good initial challenge. The plugin simply modifies a bar chart by making the bounding box a straight line at its value, and also supports both horizontal and vertical orientation. It also supports performing this on a all series or individual series in the chart.
The following example combines a chart using the topbar plugin with a traditional bar chart:
More explanation can be found in the code and on the README of the Repository.