Datafile: time_data.txt
Using the code provided in “Tutorial 03. Time-based data when plotting in gnuplot”, use the histogram style, and format the xtic labels using strftime and timecolumn.
set timefmt "%H:%S" set style fill solid 0.6 border -1 set style data histogram set style histogram clustered gap 1 plot 'data.dat' using 2:xtic(strftime('%H', timecolumn(1))), \ '' using ($2*0.5), \ '' using ($2*0.7)