! Undefined control sequence. \pgfplots@width ->0.95\textwidth -\heightof {ylabel}-\widthof {ymax}

Question

I'm trying to use pgfplots to plot a graph but I run into the following error:

! Undefined control sequence.
\pgfplots@width ->0.95\textwidth -\heightof
{ylabel}-\widthof {ymax}

Here is my document:

\documentclass[12pt]{article}

\usepackage{pgfplots}
\usepackage{pgf}            
\usepackage{tikz}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{external} 
\tikzexternalize

\begin{document}
Hello world!
\begin{figure}[htbp]
    \centering
    \begin{tikzpicture}
        \begin{axis}[%
        scale only axis,
        separate axis lines,
        ytick={-20,0,20,40,60,80,100,120,140},
        xmin=1,
        xmax=1.8,
        xlabel={$\lambda (\epsilon + 1)$ },
        ymin=-20,
        ymax=140,
        ylabel={$\sigma_{11} (kPa)$},
        legend style={legend pos=north west, legend cell align=left,align=left,draw=none},
        yticklabel style={align=right,inner sep=0pt,xshift=-0.1cm}, 
        grid=none,
        max space between ticks=40,
        major tick length=0.15cm,
        minor tick length=0.075cm,
        tick style={semithick,color=black}, 
        width=0.95\textwidth-\heightof{ylabel}-\widthof{ymax},
        height=0.5\textwidth ]
\addplot [color=black,mark size=2.5pt,only marks,mark=*,mark options={solid,fill=white!75!blue}]
  table[row sep=crcr]{%
1.00781861  0.0102718\\
1.0193275   -0.00690492\\
1.0289739   0.0408018\\
1.040467    0.0701334\\
1.05104 0.0993509\\
};
\addlegendentry{Experimental data};

\addplot [color=blue,solid,line width=1.0pt]
  table[row sep=crcr]{%
1.00781861  0.190117075896132\\
1.0193275   0.474231238225642\\
1.0289739   0.717201161857507\\
1.040467    1.01366753959628\\
1.05104 1.29424479212858\\
};
\addlegendentry{Ogden model};

\addplot [color=red,solid,line width=1.0pt]
  table[row sep=crcr]{%
1.00781861  0.777179697089887\\
1.0193275   1.92137205791344\\
1.0289739   2.88076259714836\\
1.040467    4.02448983176329\\
1.05104 5.07751266247692\\
};
\addlegendentry{Neo-Hookean model};

\addplot [color=green,solid,line width=1.0pt]
  table[row sep=crcr]{%
1.00781861  -3.7155599786856\\
1.0193275   -7.79892723072965\\
1.0289739   -10.2011747706551\\
1.040467    -12.1140706845337\\
1.05104 -13.154421182274\\
};
\addlegendentry{Mooney-Rivlin model};

        \end{axis} 
    \end{tikzpicture}
    \caption[]{Experimental data fitted to the hyperelastic models models}
    \label{figure:curve_fitting_from_data}
\end{figure}
\end{document}