set output 'ext_graph_b.ps' 
set term postscript color 'Helvetica,18'  
set grid 
set key bot right 
set ylabel 'Instrumental magnitude' 
set xlabel 'Airmass ' 
set title 'Measurements of a star on Jul 27, 2001'
plot [1.0:2.0][12.56:12.80]   './stara_jul27.dat' using ($1):($9)       with points pt 3 ps 0.5 lc 1 lw 2 t ''  ,   12.456 + 0.15366*x with lines lt 1 lc -1 lw 2  t 'slope 0.15'  
set output 
quit 
