concept every in category gnuplot

This is an excerpt from Manning's book Gnuplot in Action. Login to get full access to this book.

Whereas the index directive lets you select consecutive sets of data from a file, the every option, which we discuss now, solves a different problem. Consider the same arrangement of hosts as in the previous section, together with an auxiliary script that gathers traffic data from all hosts and dumps it into a single file. But in contrast to the previous scenario, the script now retrieves only a single day’s worth of traffic and appends it to the output file before moving on to the next host. After a few days, the output file might look like the one shown next.

Listing 4.4. Data file containing interleaved data sets (file: traffic2)

Here, each record for a single day spans three lines: one for each host. (Compare this file against the one in listing 4.3: the data in both is exactly the same!) If you want to plot the traffic for each host separately, you can use the every directive to pick up only the relevant subset of all lines. The following command, for instance, plots only the traffic for the staging host:

plot "traffic2" every 3::1 using 1:2 with lp

Using the every directive, you can control how you step through individual lines. The syntax looks similar to the syntax used for index, except that individual arguments are separated by two colons. Unfortunately, this similarity is somewhat deceiving, because the order of the arguments isn’t the same for every as it is for index:

every {int:step}[::{int:start}[::{int:end}]]

The first argument is the increment, followed (optionally) by the first and last line numbers. Line numbers are counted from zero. Don’t forget to use double colons with the every directive: single colons won’t generate an error message but will lead to strange and hard-to-predict behavior.^[3]

³I’m simplifying here. Gnuplot recognizes an additional concept known as a data block in a file: a set of consecutive lines delimited from each other using single blank lines. Data blocks are functionally redundant with data sets (delimited by double blank lines). Data blocks can be selected through additional arguments to the every directive, which are placed between the double colons. This is why it’s not illegal to use single colons in this context. If you want to know more about data blocks, check the standard gnuplot reference documentation.

You can play other tricks with the every feature. For instance, the following command skips the first two lines of every data set in the file:

plot "data" every ::2 using 1:2

This is similar to, but not quite the same as, the skip facility: skip only discards lines at the beginning of the file, not at the beginning of every data set in the file. Moreover, every only applies to data lines, whereas skip also skips comments. Feel free to devise other creative uses for the every keyword!

This is an excerpt from Manning's book Gnuplot in Action: Understanding Data with Graphs. Login to get full access to this book.

So, in summary, the index directive lets us select consecutive sets of data from a file. The every option, which we discuss next, solves a different problem.

Here, each record for a single time really spans two lines: the first line gives the temperature and the second the pressure. If we want to plot only the temperature against time, we can use the every directive to pick up only the relevant subset of all lines:

plot "data" every 2 using 1:2 with lines

Using the every directive, we can control how we step through individual lines. The syntax looks similar to the syntax used for index, except that individual arguments are separated by two colons. Unfortunately, this similarity is somewhat deceiving, because the order of the arguments isn’t the same for every as it is for index:

every {int:step}[::{int:start}[::{int:end}]]

The first argument is the increment, followed (optionally) by the first and last line number. Line numbers are counted from zero. Don’t forget to use double colons with the every directive: single colons won’t generate an error message, but will lead to strange and hard-to-predict behavior.^[1]

¹ I am simplifying here. Gnuplot recognizes an additional concept known as a data block in a file, as a set of consecutive lines delimited from each other using single blank lines. Data blocks are functionally redundant with data sets (delimited by double blank lines). Data blocks can be selected through additional arguments to the every directive, which are placed between the double colons. This is why it’s not illegal to use single colons in this context. If you want to know more about data blocks, check the standard gnuplot reference documentation.

The index and every directives can be used to pick out certain parts from a data file. But what do we do if the data itself is noisy or otherwise not fit to plot directly? That’s the job of the smooth directive, which is the topic of the next section.