untold swallow
Do you mean something like this? I adapted the first example from the pgfplots manual, sec. 4.24,
\documentclass{article}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{width=7cm,compat=1.18}
\begin{document}
\begin{tikzpicture}
\begin{axis}[legend pos=outer north east]
\addplot[draw=none] table [
y={create col/linear regression={y=Y}}, % compute a linear regression from the input table
] {
X Y
1 1
2 4
3 9
4 16
5 25
6 36
};
\xdef\slope{\pgfplotstableregressiona}
\xdef\intersect{\pgfplotstableregressionb}
\addplot[blue, domain=-10:10]{\slope*x + \intersect};
\addlegendentry{%
$\pgfmathprintnumber{\slope} \cdot x
\pgfmathprintnumber[print sign]{\intersect}$}
\end{axis}
\end{tikzpicture}
\end{document}