如何用Gnuplot绘制基于年、儒略日、HHMM的时间-第5列数据图表？

Let's walk through fixing your Gnuplot script to correctly plot the 5th column against the combined time from columns 2 (year), 3 (Julian day), and 4 (HHMM). Here are the key issues in your original code and how to fix them:

1. Data Separator Mismatch #

Your input data uses commas as separators, but Gnuplot defaults to whitespace. First, tell Gnuplot to recognize commas as the delimiter:

set datafile separator ","

2. Incorrect X-Axis Data Selection #

You're only using column 2 ($2) for the x-axis, but your time is a combination of three columns. We need to merge these into a single string that matches your timefmt specification. Use the sprintf function to format them into the "YYYY,DDD,HHMM" pattern your timefmt expects:

# Combine year, Julian day, and HHMM into one time string
(sprintf("%d,%d,%d", $2, $3, $4))

3. X-Range Format Mismatch #

Your xrange uses MM/DD/YYYY format, which doesn't align with the %Y,%j,%H%M format you set. You need to define the x-range using the same pattern as timefmt. For example:

• January 15, 2018 translates to Julian day 045 → "2018,045,0000"
• August 2, 2018 translates to Julian day 214 → "2018,214,2359"

Update the xrange line to:

set xrange ["2018,045,0000":"2018,214,2359"]

Full Working Script #

Putting it all together, here's the corrected script (I added an optional line to make x-axis labels more readable):

set datafile separator ","
set xdata time
set timefmt "%Y,%j,%H%M"
# Optional: Format x-axis to show human-readable dates/times
set format x "%Y-%m-%d\n%H:%M"
set xrange ["2018,045,0000":"2018,214,2359"]
set yrange [0:5]
plot 'du1piezometerslong050318.dat' using (sprintf("%d,%d,%d",$2,$3,$4)):5 title 'July time series' with lp lw 4 pt 7 ps 0.75

Quick Notes #

• Ensure Julian days in your data are 3-digit values (e.g., 045 instead of 45) so the %j specifier parses them correctly.
• The set format x line is optional but turns raw Julian day/time values into easy-to-read dates and times on your plot.

内容的提问来源于stack exchange，提问作者Shi Mulenga