Problem plotting

[asha g]

I have a program. When I run it with N = 10, it plots perfectly. But when I run 
it with N = 30 (everything else is the same) I get the following error message 
( Actually it ran correctly several times and then suddenly started giving this 
error) Can someone help me sort this out please.
there are four plots. One of them is plotting correctly. 

Thanks
Asha 

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
      ^
         line 31: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
    ^
         line 62: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
                                                                                
 ^         line 93: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
      ^
         line 31: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
    ^
         line 62: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
                                                                                
 ^         line 93: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
      ^
         line 155: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
    ^
         line 186: warning: Skipping data file with no valid points

gnuplot> plot "-" using ($1):($2) axes x1y1 title "rs==parent" with points 
linestyle 1 , "-" using ($1):($2) axes x1y1 title " m*==branch1" with points 
linestyle 2 , "-" using ($1):($2) axes x1y1 title "b+== branch2" with points 
linestyle 3 , "-" using ($1):($2) axes x1y1 title "" with points linestyle 4 ;
                                                                                
 ^         line 217: warning: Skipping data file with no valid points
octave:3>