Use MATLAB for graph.

Dear Monica

First concatenate your both trajectories using gmx trjcat command of gromacs. Then do your analysis. Then you will get 0-20 ns graphs.

Other wise edit the .xvg files by merging two analysis data.

Hope this will help you.

You might not have used "-append" option, that means you have two trajectories (0-10ns and 10-20ns). So, before analysis concatenate them.