I have a file that looks like
$cat myfile.dat 1 8 32 19230 1.186 3.985 1 8 64 9620 0.600 7.877 1 8 128 4810 0.312 15.136 1 8 256 2410 0.226 20.927 1 8 512 1210
You can search for the maximum and plot only that, but this is probably easier, even if it draws lots of boxes one over another:
plot "myfile.dat" using 1:6:(.1) with boxes fillstyle solid