how to draw a parameter vs time graph?

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There aren't any tools in ROS for plotting parameters over time, and there's a fairly good reason for this: ROS parameters should not change very often. If you're changing parameters to communicate data between nodes, you should probably be using a topic instead.

That said, if you have a legitimate reason for wanting to capture and plot the value of a parameter over time, you could create a simple node which checks the value of your parameter, and publishes it on a topic. (roscpp has the getParamCached() function, which would be particularly good for this.) Once you have your parameter published on a topic, you can plot it with rqt_plot and log changes using rosbag.

I believe you are looking for rqt_plot. It can plot a value versus time.