曲线拟合参数在图例中显示为 0（pgfplots、gnuplot、pgfplotstable）

Question

我在数据集上拟合了一条曲线，但参数 c 在图的拟合方程图例中始终为 0。你能帮我解释一下这是为什么吗？

我还检查了生成的parameter.dat文件，它包含c的值（第二个）：1.59029739803992 3.3388191096414e-09 0.000482995469000494

但由于某种原因，它并没有达到传说中的那样……

代码：

    \begin{figure}[h]
    \begin{center}
    \begin{tikzpicture}
    \begin{axis}[
        xlabel={$f[Hz]$},
        ylabel={\textcolor{blue}{$Z[Ohm]$}},ytick pos=left]
    \addplot [mark=x, thin, blue, only marks, mark size=1pt, forget plot] table [x=Frequency, y=Z, col sep=comma] {Data/LC.csv};        
    \addplot [blue, thin, mark = none] gnuplot [raw gnuplot] {
    r=2;
    c=3*10**(-9);
    l=490*10**-6;
    f(x)=sqrt(r**2+(1/(2*pi*x*c)-2*pi*x*l)**2);

    set xrange [100000:150000];
    set datafile separator ',';
    fit f(x) 'Data/LC.csv' u 1:2 via r,c,l;
    plot f(x);
    set print "parameters.dat";  % Open a file to save the parameters
    print r,c,l;                  % Write the parameters to file
    };       

    \addlegendentry[/pgf/number format/precision=3]{
    \pgfplotstableread{parameters.dat}\parameters
    % Open the file Gnuplot wrote

    \pgfplotstablegetelem{0}{0}\of\parameters \pgfmathsetmacro\paramr{\pgfplotsretval} % Get first element, save into \paramA
    \pgfplotstablegetelem{0}{1}\of\parameters \pgfmathsetmacro\paramc{\pgfplotsretval}
    \pgfplotstablegetelem{0}{2}\of\parameters \pgfmathsetmacro\paraml{\pgfplotsretval}
    $Z=\sqrt{\pgfmathprintnumber{\paramr}^2 + (1/(2*\pi*\pgfmathprintnumber{\paramc}*f)-2*\pi*\pgfmathprintnumber{\paraml}*f)^2}$
}

    \end{axis}
    \end{tikzpicture}
    \end{center}
    \end{figure}

Answer 1

问题似乎是语法c写的3.3e-9，这似乎混淆了 pgf/TikZ。如果您在文件中重写它，parameters.dat它0.0000000033就会起作用。要使第一个数字起作用，请激活fpu库。

有关详细信息，请查看代码中的注释。

% used PGFPlots v1.16
\documentclass[border=5pt]{standalone}
\usepackage{amsmath}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        xlabel={$f / \text{Hz}$},
        ylabel={\textcolor{blue}{$Z / \text{Ohm}$}},
        ytick pos=left,
    ]
%% you didn't provide the data file ...
%        \addplot [mark=x, thin, blue, only marks, mark size=1pt, forget plot] table [x=Frequency, y=Z, col sep=comma] {Data/LC.csv};
%        \addplot [blue, thin, mark = none] gnuplot [raw gnuplot] {
%            r=2;
%            c=3*10**(-9);
%            l=490*10**-6;
%            f(x)=sqrt(r**2+(1/(2*pi*x*c)-2*pi*x*l)**2);
%
%            set xrange [100000:150000];
%            set datafile separator ',';
%            fit f(x) 'Data/LC.csv' u 1:2 via r,c,l;
%            plot f(x);
%            set print "parameters.dat";  % Open a file to save the parameters
%            print r,c,l;                  % Write the parameters to file
%        };

        % thus, I just use a dummy `\addplot'
        \addplot coordinates {(0,0)};
        \addlegendentry[
            /pgf/number format/precision=3,
        ]{%
            % -----------------------------------------------------------------
            % activate the `fpu' library and it will work
            \pgfkeys{/pgf/fpu=true}%
            % -----------------------------------------------------------------
            \pgfplotstableread{parameters.dat}\parameters%
            % Open the file Gnuplot wrote
                \pgfplotstablegetelem{0}{0}\of\parameters%
                    \pgfmathsetmacro\paramr{\pgfplotsretval}%
                \pgfplotstablegetelem{0}{1}\of\parameters%
                    \pgfmathsetmacro\paramc{\pgfplotsretval}%
                \pgfplotstablegetelem{0}{2}\of\parameters%
                    \pgfmathsetmacro\paraml{\pgfplotsretval}%
            % and deactivate it again
            \pgfkeys{/pgf/fpu=false}%
            %
            $Z = \sqrt{\pgfmathprintnumber{\paramr}^2
                + (1/(2 \, \pi \cdot \pgfmathprintnumber{\paramc} \cdot f)
                - 2 \, \pi \cdot \pgfmathprintnumber{\paraml} \cdot f)^2}$
        }

    \end{axis}
\end{tikzpicture}
\end{document}