Simple bar graph:
set boxwidth 0.5
set style fill solid
plot "data.dat" using 1:3:xtic(2) with boxes
data.dat:
0 label 100
1 label2 450
2 "bar label" 75
If you want to style your bars differently, you can do something like:
set style line 1 lc rgb "red"
set style line 2 lc rgb "blue"
set style fill solid
set boxwidth 0.5
plot "data.dat" every ::0::0 using 1:3:xtic(2) with boxes ls 1, \
"data.dat" every ::1::2 using 1:3:xtic(2) with boxes ls 2
If you want to do multiple bars for each entry:
data.dat:
0 5
0.5 6
1.5 3
2 7
3 8
3.5 1
gnuplot:
set xtics ("label" 0.25, "label2" 1.75, "bar label" 3.25,)
set boxwidth 0.5
set style fill solid
plot 'data.dat' every 2 using 1:2 with boxes ls 1,\
'data.dat' every 2::1 using 1:2 with boxes ls 2
If you want to be tricky and use some neat gnuplot tricks:
Gnuplot has psuedo-columns that can be used as the index to color:
plot 'data.dat' using 1:2:0 with boxes lc variable
Further you can use a function to pick the colors you want:
mycolor(x) = ((x*11244898) + 2851770)
plot 'data.dat' using 1:2:(mycolor($0)) with boxes lc rgb variable
Note: you will have to add a couple other basic commands to get the same effect as the sample images.