As a first step, make sure the variable is properly mapped in the device mapper and that the device is sending the required signal.
Since there is no data in the Track Point panel, it looks like either the device is not sending it, you're using the wrong device mapper, or the variable is not mapped.
You can use the Terminal tool for debugging. Find out more in Advanced usage of this blog http://gpsgate.com/blogComment/device_mapper
and here topic.asp?TOPIC_ID=10740
The Track Point graph displays the data from track point, therefore, if the data is not available in track point, it cannot be graphed.