[Tex/LaTex] Undefined control sequence on \frac

graphsmath-modepgfplots

I'd like to generate PDF's for some ad hoc screenshots, but I'm getting an undefined control error. I tried \usepackage{amsmath} after the line \usepackage{pgfplots} but that didn't work. I thought maybe that package was required for use of \frac{x}{2}

I'm using TeXworks with the following to try and generate a generic plot of x/2:

\documentclass{article}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}[>=stealth]
    \begin{axis}[
        xmin=-10,xmax=10,
        ymin=-10,ymax=10,
        axis x line=middle,
        axis y line=middle,
        axis line style=<->,
        xlabel={$x$},
        ylabel={$y$},
        ]
        \addplot[no marks,blue,<->] expression[domain=-10:10,samples=100]{\frac{x}{2}} 
                    node[pos=0.65,anchor=south west]{$y=\frac{x}{2}$}; 
    \end{axis}
\end{tikzpicture}
\end{document}

The error is on the node line 16 I believe, but I can't tell if it's syntax or otherwise.

! Undefined control sequence.
\pgfmath@dimen@ ...men@@ #1=0.0pt\relax \pgfmath@ 

l.16 ...=0.65,anchor=south west]{$y=\frac{x}{2}$};

Please consider me very new to LaTeX. I guess I'm assuming there is a missing package, since I can successfully plot simple linear graphs that don't include \frac.

Best Answer

You should write x/2 instead of \frac{x}{2}.

In the equation of the graph, you want a mathematical expression that PGFplots can understand. The syntax for division in PGFplots is simply /.
In the label for the graph, you just want a piece of text, so y=\frac{x}{2} is fine here.

Fixed code

\documentclass{article}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}[>=stealth]
    \begin{axis}[
        xmin=-10,xmax=10,
        ymin=-10,ymax=10,
        axis x line=middle,
        axis y line=middle,
        axis line style=<->,
        xlabel={$x$},
        ylabel={$y$},
        ]
        \addplot[no marks,blue,<->] expression[domain=-10:10,samples=100]{x/2} 
                    node[pos=0.65,anchor=south west]{$y=\frac{x}{2}$}; 
    \end{axis}
\end{tikzpicture}
\end{document}

enter image description here

Related Solutions

PGFPlots – How to Plot Two Time Series with Bounds Using PGFPlots

This happens because PGFPlots only uses one "stack" per axis: You're stacking the second confidence interval on top of the first. The easiest way to fix this is probably to use the approach described in "Is there an easy way of using line thickness as error indicator in a plot?": After plotting the first confidence interval, stack the upper bound on top again, using stack dir=minus. That way, the stack will be reset to zero, and you can draw the second confidence interval in the same fashion as the first:

\documentclass{standalone}
\usepackage{pgfplots, tikz}

\usepackage{pgfplotstable}

\pgfplotstableread{
temps   y_h y_h__inf    y_h__sup    y_f y_f__inf    y_f__sup    

1   0.237340    0.135170    0.339511    0.237653    0.135482    0.339823    
2   0.561320    0.422007    0.700633    0.165871    0.026558    0.305184    
3   0.694760    0.534205    0.855314    0.074856    -0.085698   0.235411    
4   0.728306    0.560179    0.896432    0.003361    -0.164765   0.171487    
5   0.711710    0.544944    0.878477    -0.044582   -0.211349   0.122184    
6   0.671241    0.511191    0.831291    -0.073347   -0.233397   0.086703    
7   0.621177    0.471219    0.771135    -0.088418   -0.238376   0.061540    
8   0.569354    0.431826    0.706882    -0.094382   -0.231910   0.043146    
9   0.519973    0.396571    0.643376    -0.094619   -0.218022   0.028783    
10  0.475121    0.366990    0.583251    -0.091467   -0.199598   0.016664    
}{\table}

\begin{document}
\begin{tikzpicture}

    \begin{axis}
    % y_h confidence interval
    \addplot [stack plots=y, fill=none, draw=none, forget plot]   table [x=temps, y=y_h__inf]   {\table} \closedcycle;
    \addplot [stack plots=y, fill=gray!50, opacity=0.4, draw opacity=0, area legend]   table [x=temps, y expr=\thisrow{y_h__sup}-\thisrow{y_h__inf}]   {\table} \closedcycle;
    % subtract the upper bound so our stack is back at zero
    \addplot [stack plots=y, stack dir=minus, forget plot, draw=none] table [x=temps, y=y_h__sup] {\table};

    % y_f confidence interval
    \addplot [stack plots=y, fill=none, draw=none, forget plot]   table [x=temps, y=y_f__inf]   {\table} \closedcycle;
    \addplot [stack plots=y, fill=gray!50, opacity=0.4, draw opacity=0, area legend]   table [x=temps, y expr=\thisrow{y_f__sup}-\thisrow{y_f__inf}]   {\table} \closedcycle;

    % the line plots (y_h and y_f)    
    \addplot [stack plots=false, very thick,smooth,blue]  table [x=temps, y=y_h]   {\table};
    \addplot [stack plots=false, very thick,smooth,blue]  table [x=temps, y=y_f]   {\table};
    \end{axis}

\end{tikzpicture}
\end{document}

[Tex/LaTex] Apply shift to (current axis.left of origin)

I really don't know why but this works for me

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}

\pgfplotsset{compat=1.10}

\begin{document}
\begin{tikzpicture}
\begin{axis}
    \addplot {x*x};

    \draw [ultra thick, dotted, draw=brown] 
        (current axis.left of origin) -- 
        (current axis.right of origin);

    \draw [ultra thick, draw=red] 
        ($(current axis.left of origin)-(axis cs:0,-12)$) -- 
        ($(current axis.right of origin)+(axis cs:0,12)$);
\end{axis}
\end{tikzpicture}
\end{document}