Two ways：

1. Plot the function first and let gnuplot to tell the minimum x value:

   plot "inline.dat" using 1:2 with impulses linewidth 2x

   minx = GPVAL_DATA_X_MIN

   minY = GPVAL_DATA_Y_MIN

   maxx = GPVAL_DATA_X_MAX

   maxY = GPVAL_DATA_Y_MAX

   plot "inline.dat" using ($1-xmin):2 with impulses linew

2. Use external script to figure out what is the minimum x value:

   set macro (*This command is necessary)

   data = './inline.dat'    

   xmin = `sort -nk 1 data | head -n 1 | awk '{print $1}'`

   (or get the the value in the first column second row:)

 x12 = `cat @data | head -n 2 | tail -n 1 | awk '{print $1}'`

    plot "inline.dat" using ($1-xmin):2 with impulses linewidth 2

Note: the second method can not work in gnuplot in windows. It works only under pgnuplot, which works with cygwin terminal.

refenrence: http://stackoverflow.com/questions/7540614/reading-dataset-value-into-a-gnuplot-variable-start-of-x-series

http://stackoverflow.com/questions/18616063/passing-gnuplot-variables-to-a-shell-expression