CODEWITHSUNDEEP
gnuplot
England
England

Reputation: 27

Labeling Points with a date/time x axis in GNUPLOT

I have a simple dataset consisting of a date/time field (YYYY-MM-DD HH:MM) and a temperature field, taken at 1 minute intervals. I am trying to learn how to label points on a plot such than I can label the max and min temperatures.

The data is coming from an SQL server in a node.js app and the commands / data are being piped to gnuplot via STDIN with the output being a PNG. I can successfully plot the data, but now I am just trying to label the max and min temperature points on the plot, with the "coordinates" coming from an SQL query such that a min or max point would look like (x,y) = (2021-02-11 18:34, 72.57). Every command I try that should label points on the plot have no effect. And the examples that I find usually don't involve a date/time x-axis.

What is the magic ju-ju needed to be able to take an arbitrary data point and label it on the plot with a time/date x-axis? FWIW, since the data is being retrieved from an SQL server, I can format the data in whatever way makes it the easiest method needed to plot the data and more importantly, label the points that I want.

Thanks in advance for any advice!!

Justin

EDIT (per the first comment):

Here is what I am using in terms of gnuplot commands:

set term png size 1280,600
unset output
set datafile separator ","
set xdata time
set timefmt "%Y-%m-%d %H:%M"
set format x "%H:%M"
set ylabel "Temperature ˚F
set xlabel "Time
set style line 100 lt 1 lc rgb "grey" lw 0.5
set grid ls 100
set xtics border
set xtics rotate
set key off
$mydata << EOD
// this is where my code "prints" the SQL data to STDIN of gnuplot
// data is formatted: 2021-02-11 22:48,74.42
EOD
plot $mydata using 1:2 with lines

Note, I can format the data in whatever way that makes it easiest for gnuplot to process. I have found that sending YYYY-MM-DD HH:MM,TEMP has worked without issue.

I am tring to use the 'set label' command using a time/date as the x value and the temperature as the y value:

set label 1 'Maximum' at 2021-02-11 19:10,72.50

which does not produce any labels.

Here is an example plot where I want to label the max and min points:

Example Plot

Upvotes: 0

Views: 1285

Answers (1)

Ethan
Ethan

Reputation: 15093

When the x axis is time, the units are seconds. You are looking for the functions that convert between seconds (a floating point value) and a formatted time/date string.

strptime("timeformat",s) reads the time from the string s using the timeformat specifiers and converts it into seconds since the year 1970.

strftime("timeformat",t) produces a string by applying the timeformat specifiers to the time t given in seconds since the year 1970.

In the case you show, this would correspond to:

 myformat = "%Y-%m-%d %H:%M"
 set label 1 'Maximum' at strptime(myformat, "2021-02-11 19:10"), 72.50

Upvotes: 1

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