gnuplot> plot '< tail -n 40 c:/test/bitgbp1.dat'  using 3:2:((delta_v($2) > 0.0) ? 1 : -1 )with impulses lw 2 palette
                                                                                                              ^
         warning: Skipping data file with no valid points
                                                                                                                     ^
         x range is invalid