Missing or incorrect trackerId parameter for the plugin.
The numbers 8,4,7 are then generated from a tracker using a simple wiki template and a trackerlist substituted into the body content as follows
Plugin disabled
Plugin r cannot be executed.
In the above the echodebug now shows the body content both pre and post parsing so that it can be checked.
So now that you can generate output from a tracker using a trackerlist plugin, or for more flexibility using the list plugin, then a lot more options become possible with R :-)