Calculation of Van hove function in gromacs

GROMACS version: 2018.3
GROMACS modification: No
I tried calculating van hove correlation function using the gromacs module gmx van hove but all I got figure attached with this issue.
The command that I used is :
gmx vanhove -f traj_nopbc.trr -s traj.tpr -n index.ndx -om vanh_ow.xpm -or vanhove_ow_r.xvg -ot vanhove_ow_t.xvg

van_ow.xvg (996 Bytes)

I am a bit confused with the results I obtained since what is written in the synposis of gmx vanhove is that using -or one obtains the value of van hove function for one or more values of t.
Can anybody suggest where I am doing wrong…