Apply shift to (current axis.left of origin)

Question

I was trying to improve the solution at Drawing lines in a plot and give them a label? by using Jake's answer at How to access \xmin, \xmax, \ymin, \ymax from within PGFplots axis environment. So, Jake's answer produces the brown dotted line (the x-axis) with

(current axis.left of origin) -- (current axis.right of origin)

I thought a simple use of tikz's calc library would allow me to adjust the vertical position of the horizontal line. If I want a horizontal line at y=12, for example, I simply use

($(current axis.left of origin)+(axis cs: 0,12)$) -- ($(current axis.right of origin)+(axis cs: 0,12)$)

where I have applied a + (axis cs: 0,12) and same for the other coordinate.

Well, that is great in theory, but doesn't work. Instead, it produces the dashed line in red (which is only half the desired line):

Notes:

• Instead of doing coordinate calculations, I also tried applying shift={(axis cs: 0,12)} option but that had no effect on the output.

Code:

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

\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, dashed, 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}

Answer 1

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}