You can use a coordinate filter to replace the negative values with very small values, and use ymin to set the visible range to a value that's larger than the small replacement value:
\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
ymode=log,
log basis y=10,
domain=-5:5,
ymin=0.1,
y filter/.code={% If the y value is undefined (because it was the log of a negative number)
% replace it with a value that's smaller than log(ymin)
\ifx\pgfmathresult\empty\def\pgfmathresult{-2}\fi%
}]
\addplot {x^2 - 1};
\end{axis}
\end{tikzpicture}
\end{document}
After some trying I found the solution!
(I already had the correct idea in mind but this didn't work in the first run...)
Recap: A linear function has this function: a ยท x + b
And pgfplotstable lets you access
a with \pgfplotstableregressiona and
b with \pgfplotstableregressionb
of the linear regression plot.
So I used this code to access the value: (xmin was previously defined)
% get xmax (e.g. 2021-11-25)
\pgfkeysgetvalue{/pgfplots/xmax}{\xmax}
% convert xmax (date to julian, which is an integer number)
\newcount\xmaxjulian
\pgfcalendardatetojulian{\xmax}{\xmaxjulian}
\xmaxjulian=\numexpr\xmaxjulian-\xmin % remove offset
% store y value at xmax in \var
\pgfmathsetmacro{\var}{\pgfplotstableregressiona * (\the\xmaxjulian) + \pgfplotstableregressionb}
% print legend:
\addlegendentry{Forecast utilization in one year:
\luaexec{ tex.sprint ( string.format ( "\%.2f" , \var ) ) } TB}
Note:
That code \luaexec{ tex.sprint ( string.format ( "\%.2f" , \var ) ) } rounds my value to two decimal places. It only works with LuaLaTeX and you need to add \usepackage{luacode} to your code.
Note 2:
You can print the function of the linear regression with this:
Formula of linear regression:
$\pgfmathprintnumber{\pgfplotstableregressiona}\cdot x
\pgfmathprintnumber[print sign]{\pgfplotstableregressionb}$
Note 3:
I added a MWE example here: (you need LuaLaTeX in order to run it)
\documentclass{article}
\usepackage[letterpaper,top=2cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}
\usepackage{datatool}
\usepackage{luacode}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\usepackage{pgfcalendar}
\usepgfplotslibrary{dateplot}
\pgfplotsset{compat=newest}
\begin{filecontents*}{data.csv}
date, size
2021-04-01, 1.42
2021-05-01, 1.46
2021-06-01, 1.58
2021-07-01, 1.55
2021-08-01, 1.69
\end{filecontents*}
\begin{document}
\thispagestyle{empty}
\centering
\pgfplotstableread[col sep=comma]{data.csv}\loadedtable
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% add new column with Julian integer numbers
\newcount\julianday
\pgfplotstablecreatecol[
create col/assign/.code={
\pgfcalendardatetojulian{\thisrowno{0}}{\julianday}
\edef\entry{\the\julianday}
\pgfkeyslet{/pgfplots/table/create col/next content}\entry
},
]{JulianDay}{\loadedtable}
\pgfplotstablegetelem{0}{JulianDay}\of{\loadedtable}
\pgfmathtruncatemacro{\xmin}{\pgfplotsretval}
\pgfplotstablecreatecol[
expr={\thisrow{JulianDay}-\xmin},
]{JulianDayMod}{\loadedtable}
% source: https://tex.stackexchange.com/questions/367339/linear-regression-with-dates-on-x-axis-in-pgfplots
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% compute dates for xmin and xmax
% get last date in file
\DTLloaddb[%
noheader,%
keys={date,2}%
]{myDB}{data.csv}
\DTLforeach*{myDB}{\CurrentA=date}{%
\xdef\LastDate{\CurrentA}
}
\newcount\DateOfLastScan
\pgfcalendardatetojulian{\LastDate{}}{\DateOfLastScan}
\pgfcalendarjuliantodate{\DateOfLastScan}{\theyear}{\themonth}{\theday}
\year=\theyear
\month=\themonth
\day=\theday
% get date two years ago
\year=\numexpr\year-2
\edef\twoyearsago{\the\year-\ifnum\the\month<10 0\fi\the\month-\ifnum\the\day<10 0\fi\the\day}
\year=\numexpr\year+2
% get date one year in the future
\year=\numexpr\year+1
\edef\oneyearfuture{\the\year-\ifnum\the\month<10 0\fi\the\month-\ifnum\the\day<10 0\fi\the\day}
\year=\numexpr\year-1
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% print plot
\begin{tikzpicture}
\begin{axis}
[
date coordinates in=x,
ylabel={Terabyte},
xticklabel style={rotate=90,anchor=near xticklabel},
xticklabel=\scriptsize\texttt{\day-\month-\year},
yticklabel={\luaexec{ tex.sprint ( string.format ( "\%.2f" , \tick ) ) }},
grid,
tick align=inside,
width=\textwidth,
xmin=\twoyearsago{},
xmax=\oneyearfuture{},
ymin=-0.05,
legend pos = south east,
legend image post style={only marks, mark=none},
legend cell align={left},
]
% linear regression
\addplot [line width=10pt, opacity=.3, red, shorten >= -10cm, shorten <= -10cm] table [
x index=0,
% now we can use the newly created column to do the linear regression
y={create col/linear regression={
x=JulianDayMod,
y=size,
}}
] {\loadedtable};
% contents from CSV file
\addplot[thick, no marks, solid] table[col sep=comma, x index=0, y index=1]{\loadedtable};
% get y value at xmax
\pgfkeysgetvalue{/pgfplots/xmax}{\xmax}
\newcount\xmaxjulian
\pgfcalendardatetojulian{\xmax}{\xmaxjulian}
\xmaxjulian=\numexpr\xmaxjulian-\xmin
\pgfmathsetmacro{\var}{\pgfplotstableregressiona * (\the\xmaxjulian) + \pgfplotstableregressionb}
% add legends
\addlegendentry{Forecast utilization in one year: \textbf{\luaexec{%
tex.sprint ( string.format ( "\%.2f" , \var ) ) } TB}}
\addlegendentry{Formula of linear regression:
$\pgfmathprintnumber{\pgfplotstableregressiona}\cdot x
\pgfmathprintnumber[print sign]{\pgfplotstableregressionb}$
}
% add invisible point at end of linear regression to keep the end of line inside of the plot
\addplot[forget plot,draw=none] coordinates {(\xmax,\var)};
\end{axis}
\end{tikzpicture}
\end{document}
Result:
Best Answer
If I understand correctly, you want all values between -40 dB and +20 dB to be plotted in the positive domain, with -40 dB being the origin of the pot. For that, you need to transform your data and then backtransform the axis labels, see Negative y value in polar plot.
For having the y axis outside the plot, you can use the approach from Gonzalo Medina's answer to Setting axis line offset?.