Understanding the use of the keyword every - gnuplot

Question

My question regards the keyword every that is used to sample an input data file (i.e., .csv, .dat etc.). I am reading the documentation of the keyword that says the following:
plot 'file' every {<point_incr>}
{:{<block_incr>}
{:{<start_point>}
{:{<start_block>}
{:{<end_point>}
{:<end_block>}}}}}
The thing is I cannot completely comprehend how to adapt this to a data set. For instance, if I have some dummy data that I wish to use to create a bar chart for example and the data are the following
# first bars group
#x axis #y axis
0 2
0.2 3
0.4 4
0.6 5
0.8 6
#second bars group
1 1
1.2 2
1.4 3
1.6 4
1.8 5
#etc.
3 10
3.2 20
3.4 30
3.6 40
3.8 50
4 20
4.2 30
4.4 40
4.6 50
4.8 60
And lets say that I want to create four bar clusters from the data. One for every block. How can I use the syntax of the keyword? Could someone give me some examples to better understand the use of it? Thank you in advance

Answer 1

As you've found, the every keyword allows you to cherry-pick a subset of single-newline-separated points and double-newline-separated blocks from your datafile. Your example datafile shows 20 points divided into 4 blocks.
So to plot the first block (indexed 0 in gnuplot), you only need to specify the end block, and use the default values for the other every parameters. Try:
plot 'data.txt' every :::::0 with boxes
It seems your goal is to plot each block with separate styling. Here's how you could do that with a few extra styling commands. (Note my use of gnuplot's shorthand for some keywords.)
set key left top
set boxwidth 0.2
p 'data.txt' ev :::0::0 w boxes t 'first',\
'data.txt' ev :::1::1 w boxes t 'second',\
'data.txt' ev :::2::2 w boxes t 'third',\
'data.txt' ev :::3::3 w boxes t 'fourth'

Answer 2

From help every:
The data points to be plotted are selected according to a loop from
<start_point> to <end_point> with increment <point_incr> and the
blocks according to a loop from <start_block> to <end_block> with
increment <block_incr>.
This should be pretty clear, however, you have to know if blocks are separated by two (or more) empty lines, you have to address them differently. Check help index. To my opinion the documentation is a bit confusing about datablock, (sub-)block, dataset, etc...
Check the following example. I assume this is not your final graph, but still needs some tuning. Depending on your detailed requirements you also might want to check help histograms.
For example every :::i::i will plot all datapoints in in block i, i.e. from block i to block i.
Code:
### plotting using "every"
reset session
$Data <<EOD
# first bars group
#x axis #y axis
0 2
0.2 3
0.4 4
0.6 5
0.8 6
#second bars group
1 1
1.2 2
1.4 3
1.6 4
1.8 5
#etc.
3 10
3.2 20
3.4 30
3.6 40
3.8 50
4 20
4.2 30
4.4 40
4.6 50
4.8 60
EOD
set key top left
set boxwidth 0.2
set key out noautotitles
set style fill solid 0.3
set yrange [:70]
plot for [i=0:3] $Data u 1:2 every :::i::i w boxes
### end of code
Result: