Drawing lines in a plot and give them a label?

Question

I am on my first adventure in plotting with LaTeX. I managed to get my bar chart plotted. I have two groups of values for which I want to show the average. I intend to put a horizontal bar over the bar chart and let it indicate the average values. Drawing is unfortunately not coupled to scaling or similar. In the moment I have to change something in the scaling/labeling everything will be out of its place. Is there a best practice way of drawing lines (or arrows) that are corresponding to data values of the underlying chart?

Answer 1

I am assuming that you are having trouble using a tikz \draw within a \pgfplot's axis environment. In this case you need to use the axis cs coordinate system as the dotted line in red illustrates:

Notes:

• To add a label to this line you can use a node as part of the draw.
• The positioning of the label is covered at Moving a label along the path.
• Thanks to Jake's answer at Apply shift to (current axis.left of origin), the second MWE below should work without requiring specification of the minimum and maximum x values. Only the y value needs to be specified in the \VerticalPos definition.

Code: Specify both minimum and maximum x values

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}
    \addplot {x*x};
    \draw [ultra thick, dotted, draw=red] 
        (axis cs: -6,12) -- (axis cs: 6,12)
        node[pos=0.5, above] {$y=12$};
\end{axis}
\end{tikzpicture}
\end{document}

---

Code: Automatically determine both minimum and maximum x values

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\begin{axis}
    \addplot {x*x};
\newcommand*{\VerticalPos}{12}% Desired vertical postion
\coordinate (Left)  at ($(current axis.left of origin) +(axis direction cs: 0,\VerticalPos)$);
\coordinate (Right) at ($(current axis.right of origin)+(axis direction cs: 0,\VerticalPos)$);

\draw [ultra thick, dotted, draw=red] 
    (Left) -- (Right)
    node[pos=0.5, above] {$y=\VerticalPos$};

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