1

This is my plot:

my plot

I would like to add the best fit line in correspondence with the first straight section (from 16:23 to 16:33). The equation of the line is y = 0.022x + 19.018. This is my attempt

\documentclass{standalone}

    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage{amsmath,amssymb,amsfonts,systeme} 
    \usepackage{color}                    
    \usepackage{array}
    \usepackage{tikz}
    \usepackage{tkz-euclide}
    \usetkzobj{all}
    \usetikzlibrary{arrows.meta}
    \usepackage{siunitx}
    \usepackage{pgfplots}
    \usepgfplotslibrary{dateplot, statistics}
    \usepackage{filecontents}

\begin{filecontents*}{ab.txt}
date, value
2019-04-17 16:23:56, 18.94
2019-04-17 16:24:01, 18.96
2019-04-17 16:24:06, 18.97
2019-04-17 16:24:11, 18.98
2019-04-17 16:24:16, 18.99
2019-04-17 16:24:21,    19
2019-04-17 16:24:26, 19.01
2019-04-17 16:24:31, 19.01
2019-04-17 16:24:36, 19.02
2019-04-17 16:24:41, 19.03
2019-04-17 16:24:46, 19.03
2019-04-17 16:24:51, 19.04
2019-04-17 16:24:56, 19.04
2019-04-17 16:25:02, 19.05
2019-04-17 16:25:07, 19.05
2019-04-17 16:25:11, 19.05
2019-04-17 16:25:16, 19.06
2019-04-17 16:25:21, 19.06
2019-04-17 16:25:27, 19.06
2019-04-17 16:25:32, 19.07
2019-04-17 16:25:37, 19.07
2019-04-17 16:25:42, 19.07
2019-04-17 16:25:46, 19.08
2019-04-17 16:25:52, 19.08
2019-04-17 16:25:57, 19.08
2019-04-17 16:26:02, 19.09
2019-04-17 16:26:07, 19.09
2019-04-17 16:26:12, 19.09
2019-04-17 16:26:17, 19.09
2019-04-17 16:26:22,  19.1
2019-04-17 16:26:27,  19.1
2019-04-17 16:26:32,  19.1
2019-04-17 16:26:37, 19.11
2019-04-17 16:26:42, 19.11
2019-04-17 16:26:47, 19.11
2019-04-17 16:26:52, 19.11
2019-04-17 16:26:57, 19.12
2019-04-17 16:27:02, 19.12
2019-04-17 16:27:07, 19.12
2019-04-17 16:27:12, 19.12
2019-04-17 16:27:17, 19.12
2019-04-17 16:27:22, 19.13
2019-04-17 16:27:27, 19.13
2019-04-17 16:27:32, 19.13
2019-04-17 16:27:37, 19.13
2019-04-17 16:27:42, 19.14
2019-04-17 16:27:47, 19.14
2019-04-17 16:27:52, 19.14
2019-04-17 16:27:57, 19.14
2019-04-17 16:28:02, 19.14
2019-04-17 16:28:07, 19.15
2019-04-17 16:28:12, 19.15
2019-04-17 16:28:17, 19.15
2019-04-17 16:28:22, 19.15
2019-04-17 16:28:27, 19.16
2019-04-17 16:28:32, 19.16
2019-04-17 16:28:37, 19.16
2019-04-17 16:28:42, 19.16
2019-04-17 16:28:47, 19.16
2019-04-17 16:28:52, 19.17
2019-04-17 16:28:57, 19.17
2019-04-17 16:29:02, 19.17
2019-04-17 16:29:07, 19.17
2019-04-17 16:29:12, 19.17
2019-04-17 16:29:17, 19.17
2019-04-17 16:29:22, 19.17
2019-04-17 16:29:27, 19.18
2019-04-17 16:29:32, 19.18
2019-04-17 16:29:37, 19.18
2019-04-17 16:29:42, 19.18
2019-04-17 16:29:47, 19.18
2019-04-17 16:29:52, 19.19
2019-04-17 16:29:57, 19.19
2019-04-17 16:30:02, 19.19
2019-04-17 16:30:07, 19.19
2019-04-17 16:30:12,  19.2
2019-04-17 16:30:17,  19.2
2019-04-17 16:30:22,  19.2
2019-04-17 16:30:27,  19.2
2019-04-17 16:30:32,  19.2
2019-04-17 16:30:37,  19.2
2019-04-17 16:30:42,  19.2
2019-04-17 16:30:47, 19.21
2019-04-17 16:30:52, 19.21
2019-04-17 16:30:57, 19.21
2019-04-17 16:31:02, 19.21
2019-04-17 16:31:07, 19.21
2019-04-17 16:31:12, 19.21
2019-04-17 16:31:17, 19.21
2019-04-17 16:31:22, 19.22
2019-04-17 16:31:27, 19.22
2019-04-17 16:31:32, 19.22
2019-04-17 16:31:37, 19.22
2019-04-17 16:31:42, 19.23
2019-04-17 16:31:47, 19.23
2019-04-17 16:31:52, 19.23
2019-04-17 16:31:57, 19.23
2019-04-17 16:32:02, 19.23
2019-04-17 16:32:07, 19.23
2019-04-17 16:32:12, 19.23
2019-04-17 16:32:17, 19.23
2019-04-17 16:32:22, 19.24
2019-04-17 16:32:27, 19.24
2019-04-17 16:32:32, 19.24
2019-04-17 16:32:37, 19.24
2019-04-17 16:32:42, 19.24
2019-04-17 16:32:47, 19.24
2019-04-17 16:32:52, 19.25
2019-04-17 16:32:57, 19.25
2019-04-17 16:33:02, 19.25
2019-04-17 16:33:07, 19.25
2019-04-17 16:33:12, 19.25
2019-04-17 16:33:17, 19.25
2019-04-17 16:33:22, 19.25
2019-04-17 16:33:27, 19.26
2019-04-17 16:33:32, 19.26
2019-04-17 16:33:37, 19.26
2019-04-17 16:33:42, 19.28
2019-04-17 16:33:47, 19.45
2019-04-17 16:33:52, 19.73
2019-04-17 16:33:58, 20.18
2019-04-17 16:34:02, 20.44
2019-04-17 16:34:07, 20.77
2019-04-17 16:34:15, 21.11
2019-04-17 16:34:17, 21.19
2019-04-17 16:34:22,  21.3
2019-04-17 16:34:27,  21.4
2019-04-17 16:34:32, 21.51
2019-04-17 16:34:37, 21.58
2019-04-17 16:34:42, 21.65
2019-04-17 16:34:47,  21.7
2019-04-17 16:34:52, 21.75
2019-04-17 16:34:57, 21.79
2019-04-17 16:35:02, 21.81
2019-04-17 16:35:07, 21.84
2019-04-17 16:35:12, 21.87
2019-04-17 16:35:17, 21.89
2019-04-17 16:35:22, 21.91
2019-04-17 16:35:27, 21.93
2019-04-17 16:35:32, 21.94
2019-04-17 16:35:38, 21.96
2019-04-17 16:35:42, 21.97
2019-04-17 16:35:47, 21.99
2019-04-17 16:35:52,    22
2019-04-17 16:35:58, 22.01
2019-04-17 16:36:03, 22.02
2019-04-17 16:36:08, 22.03
2019-04-17 16:36:13, 22.04
2019-04-17 16:36:17, 22.05
2019-04-17 16:36:23, 22.06
2019-04-17 16:36:28, 22.07
2019-04-17 16:36:33, 22.08
2019-04-17 16:36:38, 22.09
2019-04-17 16:36:43, 22.09
2019-04-17 16:36:48,  22.1
2019-04-17 16:36:53, 22.11
2019-04-17 16:36:58, 22.12
2019-04-17 16:37:03, 22.12
2019-04-17 16:37:08, 22.13
2019-04-17 16:37:13, 22.14
2019-04-17 16:37:18, 22.15
2019-04-17 16:37:23, 22.15
2019-04-17 16:37:28, 22.16
2019-04-17 16:37:33, 22.16
2019-04-17 16:37:38, 22.17
2019-04-17 16:37:43, 22.17
2019-04-17 16:37:48, 22.18
2019-04-17 16:37:53, 22.18
2019-04-17 16:37:58, 22.19
2019-04-17 16:38:03, 22.19
2019-04-17 16:38:08,  22.2
2019-04-17 16:38:13,  22.2
2019-04-17 16:38:18,  22.2
2019-04-17 16:38:23, 22.21
2019-04-17 16:38:28, 22.21
2019-04-17 16:38:33, 22.22
2019-04-17 16:38:38, 22.23
2019-04-17 16:38:43, 22.22
2019-04-17 16:38:48, 22.23
2019-04-17 16:38:53, 22.23
2019-04-17 16:38:58, 22.24
2019-04-17 16:39:03, 22.24
2019-04-17 16:39:08, 22.24
2019-04-17 16:39:13, 22.25
2019-04-17 16:39:18, 22.25
2019-04-17 16:39:23, 22.25
2019-04-17 16:39:28, 22.26
2019-04-17 16:39:33, 22.26
2019-04-17 16:39:41, 22.26
2019-04-17 16:39:43, 22.26
2019-04-17 16:39:48, 22.27
2019-04-17 16:39:53, 22.27
2019-04-17 16:39:58, 22.27
2019-04-17 16:40:03, 22.27
2019-04-17 16:40:08, 22.28
2019-04-17 16:40:13, 22.28
2019-04-17 16:40:18, 22.28
2019-04-17 16:40:23, 22.28
2019-04-17 16:40:28, 22.29
2019-04-17 16:40:33, 22.29
2019-04-17 16:40:38,  22.3
2019-04-17 16:40:43, 22.29
2019-04-17 16:40:48,  22.3
2019-04-17 16:40:53,  22.3
2019-04-17 16:40:58,  22.3
2019-04-17 16:41:03,  22.3
2019-04-17 16:41:08, 22.31
2019-04-17 16:41:13, 22.31
2019-04-17 16:41:18, 22.31
2019-04-17 16:41:23, 22.31
2019-04-17 16:41:28, 22.31
2019-04-17 16:41:33, 22.31
2019-04-17 16:41:38, 22.32
2019-04-17 16:41:43, 22.32
2019-04-17 16:41:48, 22.32
\end{filecontents*}

\begin{document}
\begin{tikzpicture}
  \begin{axis}[xlabel=t/s, 
  ylabel=$\theta / \si{\celsius}$,
  grid, grid style=dashed,
   date coordinates in=x, 
   table/col sep=comma, 
   date ZERO=2019-04-17 16:23:56, 
   xticklabel=\hour:\minute, 
   xticklabel style={rotate=90, anchor=near xticklabel},
   ymin=18,
   ymax=23.5,
   xmax=2019-04-17 16:43:56,
   xtick={2019-04-17 16:25:56,2019-04-17 16:27:56,2019-04-17 16:29:56,2019-04-17 16:31:56,2019-04-17 16:33:56,2019-04-17 16:35:56,2019-04-17 16:37:56,
   2019-04-17 16:39:56,2019-04-17 16:41:56},
   extra x ticks={2019-04-17 16:23:56},
   extra y ticks={18,18.5,19.5,20.5,21.5,22.5},
   axis lines = middle,
   set layers,
   axis on top,
   x label style={at={(1,0)},right},
   y label style={at={(0,1)},above},
   style={thick},
   ]
  \addplot+[red,no markers,smooth] table[x=date,y=value] {ab.txt};
  \addplot [yellow] table[x=date,y=value] {0.022*x + 19.018};
  \end{axis}
\end{tikzpicture}
\end{document}

And this is the error message:

! Package pgfplots Error: Could not read table file '"0.022x + 19.018"' in 'search path=.'. In case you intended to provide inline data: maybe TeX screwed upyour end-of-lines? Try 'row sep=crcr' and terminate your lines with '\' (refer to the pgfplotstable manual for details).See the pgfplots package documentation for explanation.Type H for immediate help.... ...table[x=date,y=value] {0.022x + 19.018};

Can you help me?

Thank you in advance

  • Well, when you say \addplot [yellow] table[x=date,y=value] {0.022*x + 19.018}; the stuff in {} is supposed to be a filename or a previously loaded table. To plot y = f(x), do \addplot [yellow] {0.022*x + 19.018};. However, that doesn't work because of the date coordinates. What does x represent though? Seconds/minutes/hours/days? – Torbjørn T. May 3 at 16:45
  • The x represent the time in hh:mm:ss format – user3204810 May 3 at 16:55
  • Ehm, how does that work? When you calculated the 0.22, what was the unit time? – Torbjørn T. May 3 at 17:00
  • Unfortunately that data table was automatically generated in .xls file by software during the experience. The best would be to consider the first value 2019-04-17 16:23:56 ad the time t = 0 and calculate the time span, in seconds, that elapses at the following dates/times – user3204810 May 3 at 17:21
  • I think minutes were used by whatever software it was. – Torbjørn T. May 3 at 17:49
1

I don't have any brilliant solutions, but it is possible to get what you're after. After running some numbers in Octave I think the expression for the linear fit you give are in terms of minutes. So what you could do is to set xmin and xmax to specific values, and calculate the number of minutes the axis spans. After that, add a second axis environment where you plot the line. Here set xmin=0 and xmax to the number of minutes the first axis covers. Remember that when you do this you need to set ymin/ymax to the same for both axes. Use hide axis for the second axis, as you're only interested in the plot line.

\documentclass{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{siunitx}
\usepackage{pgfplots}
\usepgfplotslibrary{dateplot, statistics}
\usepackage{filecontents}

\begin{filecontents*}{ab.txt}
date, value
2019-04-17 16:23:56, 18.94
2019-04-17 16:24:01, 18.96
2019-04-17 16:24:06, 18.97
2019-04-17 16:24:11, 18.98
2019-04-17 16:24:16, 18.99
2019-04-17 16:24:21,    19
2019-04-17 16:24:26, 19.01
2019-04-17 16:24:31, 19.01
2019-04-17 16:24:36, 19.02
2019-04-17 16:24:41, 19.03
2019-04-17 16:24:46, 19.03
2019-04-17 16:24:51, 19.04
2019-04-17 16:24:56, 19.04
2019-04-17 16:25:02, 19.05
2019-04-17 16:25:07, 19.05
2019-04-17 16:25:11, 19.05
2019-04-17 16:25:16, 19.06
2019-04-17 16:25:21, 19.06
2019-04-17 16:25:27, 19.06
2019-04-17 16:25:32, 19.07
2019-04-17 16:25:37, 19.07
2019-04-17 16:25:42, 19.07
2019-04-17 16:25:46, 19.08
2019-04-17 16:25:52, 19.08
2019-04-17 16:25:57, 19.08
2019-04-17 16:26:02, 19.09
2019-04-17 16:26:07, 19.09
2019-04-17 16:26:12, 19.09
2019-04-17 16:26:17, 19.09
2019-04-17 16:26:22,  19.1
2019-04-17 16:26:27,  19.1
2019-04-17 16:26:32,  19.1
2019-04-17 16:26:37, 19.11
2019-04-17 16:26:42, 19.11
2019-04-17 16:26:47, 19.11
2019-04-17 16:26:52, 19.11
2019-04-17 16:26:57, 19.12
2019-04-17 16:27:02, 19.12
2019-04-17 16:27:07, 19.12
2019-04-17 16:27:12, 19.12
2019-04-17 16:27:17, 19.12
2019-04-17 16:27:22, 19.13
2019-04-17 16:27:27, 19.13
2019-04-17 16:27:32, 19.13
2019-04-17 16:27:37, 19.13
2019-04-17 16:27:42, 19.14
2019-04-17 16:27:47, 19.14
2019-04-17 16:27:52, 19.14
2019-04-17 16:27:57, 19.14
2019-04-17 16:28:02, 19.14
2019-04-17 16:28:07, 19.15
2019-04-17 16:28:12, 19.15
2019-04-17 16:28:17, 19.15
2019-04-17 16:28:22, 19.15
2019-04-17 16:28:27, 19.16
2019-04-17 16:28:32, 19.16
2019-04-17 16:28:37, 19.16
2019-04-17 16:28:42, 19.16
2019-04-17 16:28:47, 19.16
2019-04-17 16:28:52, 19.17
2019-04-17 16:28:57, 19.17
2019-04-17 16:29:02, 19.17
2019-04-17 16:29:07, 19.17
2019-04-17 16:29:12, 19.17
2019-04-17 16:29:17, 19.17
2019-04-17 16:29:22, 19.17
2019-04-17 16:29:27, 19.18
2019-04-17 16:29:32, 19.18
2019-04-17 16:29:37, 19.18
2019-04-17 16:29:42, 19.18
2019-04-17 16:29:47, 19.18
2019-04-17 16:29:52, 19.19
2019-04-17 16:29:57, 19.19
2019-04-17 16:30:02, 19.19
2019-04-17 16:30:07, 19.19
2019-04-17 16:30:12,  19.2
2019-04-17 16:30:17,  19.2
2019-04-17 16:30:22,  19.2
2019-04-17 16:30:27,  19.2
2019-04-17 16:30:32,  19.2
2019-04-17 16:30:37,  19.2
2019-04-17 16:30:42,  19.2
2019-04-17 16:30:47, 19.21
2019-04-17 16:30:52, 19.21
2019-04-17 16:30:57, 19.21
2019-04-17 16:31:02, 19.21
2019-04-17 16:31:07, 19.21
2019-04-17 16:31:12, 19.21
2019-04-17 16:31:17, 19.21
2019-04-17 16:31:22, 19.22
2019-04-17 16:31:27, 19.22
2019-04-17 16:31:32, 19.22
2019-04-17 16:31:37, 19.22
2019-04-17 16:31:42, 19.23
2019-04-17 16:31:47, 19.23
2019-04-17 16:31:52, 19.23
2019-04-17 16:31:57, 19.23
2019-04-17 16:32:02, 19.23
2019-04-17 16:32:07, 19.23
2019-04-17 16:32:12, 19.23
2019-04-17 16:32:17, 19.23
2019-04-17 16:32:22, 19.24
2019-04-17 16:32:27, 19.24
2019-04-17 16:32:32, 19.24
2019-04-17 16:32:37, 19.24
2019-04-17 16:32:42, 19.24
2019-04-17 16:32:47, 19.24
2019-04-17 16:32:52, 19.25
2019-04-17 16:32:57, 19.25
2019-04-17 16:33:02, 19.25
2019-04-17 16:33:07, 19.25
2019-04-17 16:33:12, 19.25
2019-04-17 16:33:17, 19.25
2019-04-17 16:33:22, 19.25
2019-04-17 16:33:27, 19.26
2019-04-17 16:33:32, 19.26
2019-04-17 16:33:37, 19.26
2019-04-17 16:33:42, 19.28
2019-04-17 16:33:47, 19.45
2019-04-17 16:33:52, 19.73
2019-04-17 16:33:58, 20.18
2019-04-17 16:34:02, 20.44
2019-04-17 16:34:07, 20.77
2019-04-17 16:34:15, 21.11
2019-04-17 16:34:17, 21.19
2019-04-17 16:34:22,  21.3
2019-04-17 16:34:27,  21.4
2019-04-17 16:34:32, 21.51
2019-04-17 16:34:37, 21.58
2019-04-17 16:34:42, 21.65
2019-04-17 16:34:47,  21.7
2019-04-17 16:34:52, 21.75
2019-04-17 16:34:57, 21.79
2019-04-17 16:35:02, 21.81
2019-04-17 16:35:07, 21.84
2019-04-17 16:35:12, 21.87
2019-04-17 16:35:17, 21.89
2019-04-17 16:35:22, 21.91
2019-04-17 16:35:27, 21.93
2019-04-17 16:35:32, 21.94
2019-04-17 16:35:38, 21.96
2019-04-17 16:35:42, 21.97
2019-04-17 16:35:47, 21.99
2019-04-17 16:35:52,    22
2019-04-17 16:35:58, 22.01
2019-04-17 16:36:03, 22.02
2019-04-17 16:36:08, 22.03
2019-04-17 16:36:13, 22.04
2019-04-17 16:36:17, 22.05
2019-04-17 16:36:23, 22.06
2019-04-17 16:36:28, 22.07
2019-04-17 16:36:33, 22.08
2019-04-17 16:36:38, 22.09
2019-04-17 16:36:43, 22.09
2019-04-17 16:36:48,  22.1
2019-04-17 16:36:53, 22.11
2019-04-17 16:36:58, 22.12
2019-04-17 16:37:03, 22.12
2019-04-17 16:37:08, 22.13
2019-04-17 16:37:13, 22.14
2019-04-17 16:37:18, 22.15
2019-04-17 16:37:23, 22.15
2019-04-17 16:37:28, 22.16
2019-04-17 16:37:33, 22.16
2019-04-17 16:37:38, 22.17
2019-04-17 16:37:43, 22.17
2019-04-17 16:37:48, 22.18
2019-04-17 16:37:53, 22.18
2019-04-17 16:37:58, 22.19
2019-04-17 16:38:03, 22.19
2019-04-17 16:38:08,  22.2
2019-04-17 16:38:13,  22.2
2019-04-17 16:38:18,  22.2
2019-04-17 16:38:23, 22.21
2019-04-17 16:38:28, 22.21
2019-04-17 16:38:33, 22.22
2019-04-17 16:38:38, 22.23
2019-04-17 16:38:43, 22.22
2019-04-17 16:38:48, 22.23
2019-04-17 16:38:53, 22.23
2019-04-17 16:38:58, 22.24
2019-04-17 16:39:03, 22.24
2019-04-17 16:39:08, 22.24
2019-04-17 16:39:13, 22.25
2019-04-17 16:39:18, 22.25
2019-04-17 16:39:23, 22.25
2019-04-17 16:39:28, 22.26
2019-04-17 16:39:33, 22.26
2019-04-17 16:39:41, 22.26
2019-04-17 16:39:43, 22.26
2019-04-17 16:39:48, 22.27
2019-04-17 16:39:53, 22.27
2019-04-17 16:39:58, 22.27
2019-04-17 16:40:03, 22.27
2019-04-17 16:40:08, 22.28
2019-04-17 16:40:13, 22.28
2019-04-17 16:40:18, 22.28
2019-04-17 16:40:23, 22.28
2019-04-17 16:40:28, 22.29
2019-04-17 16:40:33, 22.29
2019-04-17 16:40:38,  22.3
2019-04-17 16:40:43, 22.29
2019-04-17 16:40:48,  22.3
2019-04-17 16:40:53,  22.3
2019-04-17 16:40:58,  22.3
2019-04-17 16:41:03,  22.3
2019-04-17 16:41:08, 22.31
2019-04-17 16:41:13, 22.31
2019-04-17 16:41:18, 22.31
2019-04-17 16:41:23, 22.31
2019-04-17 16:41:28, 22.31
2019-04-17 16:41:33, 22.31
2019-04-17 16:41:38, 22.32
2019-04-17 16:41:43, 22.32
2019-04-17 16:41:48, 22.32
\end{filecontents*}

\begin{document}
\begin{tikzpicture}
% set common options for the two axes, such as y-range
% if you want any scaling, do that here as well
\pgfplotsset{ 
  ymin=18,
  ymax=23.5
  }

\begin{axis}[xlabel=t/s, 
   ylabel=$\theta / \si{\celsius}$,
   grid, grid style=dashed,
   date coordinates in=x, 
   table/col sep=comma, 
   date ZERO=2019-04-17 16:23:56, 
   xticklabel=\hour:\minute, 
   xticklabel style={rotate=90, anchor=near xticklabel},
   xmin=2019-04-17 16:23, % <-- added
   xmax=2019-04-17 16:42, % <-- edited => 19minutes range on axis
   xtick={2019-04-17 16:25:56,2019-04-17 16:27:56,2019-04-17 16:29:56,2019-04-17 16:31:56,2019-04-17 16:33:56,2019-04-17 16:35:56,2019-04-17 16:37:56,
   2019-04-17 16:39:56,2019-04-17 16:41:56},
   extra x ticks={2019-04-17 16:23:56},
   extra y ticks={18,18.5,19.5,20.5,21.5,22.5},
   axis lines = middle,
   set layers,
   axis on top,
   x label style={at={(1,0)},right},
   y label style={at={(0,1)},above},
   style={thick},
   ]
  \addplot+[red,no markers,smooth] table[x=date,y=value] {ab.txt};

  \end{axis}
  \begin{axis}[
    hide axis,
    xmin=0,
    xmax=19, % <-- 19 units range on this axis
    domain=0:10, % plot line for first 10 minutes
    samples=2
    ]
   \addplot [dashed,black] {0.022*x + 19.018};
   \end{axis}
\end{tikzpicture}
\end{document}

enter image description here

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