First, the '-volreg' option can only be used when you input exactly the '[1..6]' columns (roll pitch yaw dS dL dP). If you input anything else, the labels will be wrong (as you've seen). You can only get what you want by putting the desired labels in manually, as in

  1dplot ... -ynames dS dL dP - Task3dmotion.1D'[4..6]'

where the standalone '-' means 'this is the end of the -ynames list'.

As far as the graph you show, that is clearly all 9 columns of the file. Since you limit the y-axis range to be -4..4, the first column ('n') will go out of range at row 5. The last 2 columns ('rmsold', 'rmsnew') are probably out of this range entirely, so nothing is shown for them.