strange coordinates in after end axis

Question

I found some answer here about how to get ymin/ymax automatically calculated by pgfplots. One can add to axis settings after end axis/.code={\pgfmathsetmacro{\hi}{\pgfkeysvalueof{/pgfplots/ymax}}} and similarly for ymin. My MWE is:

\RequirePackage{luatex85}
\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usetikzlibrary{calc}

\begin{document}

\def\xmi{0}
\def\xma{3.5}

\begin{tikzpicture}
\begin{axis}[xlabel={x},ylabel={y}, no markers, samples=100, clip mode=individual, xmin=\xmi, xmax=\xma, restrict x to domain=\xmi:\xma,
after end axis/.code={
\pgfmathsetmacro{\hi}{\pgfkeysvalueof{/pgfplots/ymax}};
\pgfmathsetmacro{\lo}{\pgfkeysvalueof{/pgfplots/ymin}};
\xdef\hhi{1.2*\hi}; % tried this, but it gives error further in node{test}
\fill[green] ($(\xmi,\hi + 0.05*\hi - 0.05*\lo)$) rectangle ($(\xma,\hi + 0.1*\hi - 0.1*\lo)$);
\node at (\xma/2,\hi) {node1};
}
]

\addplot {6*sin(deg(x))};

\node at (2,8) {text};

%\node at ($(\xma/2, \hhi)$) {test}; % this gives error

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

I see no green rectangle above figure, and {node1} is in wrong position. Also commented node {test} gives error. Why sin don't start from x=0 exactly? I saw also that using restrict ... may change whole picture. Where am I wrong? Thank you.

Answer 1

Seems you actually need axis cs to use axis coordinates in after end axis, even with compat=1.15. You do not however need the dollar signs in coordinates, they are not required to do arithmetic in the components of a coordinate, like you do. (Read section 13.5 in the TikZ manual to see what you can use that syntax for.)

The green rectangle does show up in your code as well. I suspect that the components of the coordinates are read as lengths in points. Hence, the the end point of the rectangle, for example, is placed at approximately (3.5pt, 7.8pt).

Regarding the \xdefed macro, I think you're trying to use it at a time when it is not yet been defined, \hhi will only be available after \end{axis}. (Where it will expand to 1.2*6.80955100000, and not the resulting value of that calculation, of course.)

Finally, the reason your sine curve doesn't start at zero, is a very common cause of confusion. The default domain is -5:5, and with 100 samples you get a point at x=-0.05, and the next one at x=0.05. Because of your restrict x to domain setting, the one at -0.05 is discarded (well, set to -inf, actually), and the curve thus starts at 0.05. Here you probably want to use domain=\xmi:\xma instead of restrict to domain.

\RequirePackage{luatex85}
\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}

\newcommand\xmi{0}
\newcommand\xma{3.5}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
  xlabel={x},
  ylabel={y},
  no markers,
  samples=100,
  clip=false,
  xmin=\xmi,
  xmax=\xma,
  domain=\xmi:\xma,
  after end axis/.code={
    \pgfmathsetmacro{\hi}{\pgfkeysvalueof{/pgfplots/ymax}};
    \pgfmathsetmacro{\lo}{\pgfkeysvalueof{/pgfplots/ymin}};
    % note axis cs: in the coordinates in the next two lines
    \fill[green,ultra thick] (axis cs:\xmi,\hi+0.05*\hi-0.05*\lo) rectangle (axis cs:\xma,\hi+0.1*\hi-0.1*\lo);
    \node at (axis cs:\xma/2,\hi) {node1};
  }
]

\addplot {6*sin(deg(x))};

\node at (2,8) {text};

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

Alternative

A couple of different approaches to drawing a rectangle like that, using the features of the calc library. This relies on giving the axis a name, so that its anchors can be accessed.

\RequirePackage{luatex85}
\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usetikzlibrary{calc}

\newcommand\xmi{0}
\newcommand\xma{3.5}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
  % note addition of name=ax
  name=ax,
  xlabel={x},
  ylabel={y},
  no markers,
  samples=100,
  xmin=\xmi,
  xmax=\xma,
  domain=\xmi:\xma,
]

\addplot {6*sin(deg(x))};

\node at (2,8) {text};
\end{axis}

\path
 % the let syntax requires \usetikzlibrary{calc}
 let
  % with \p1=(ax.south west), the x-component of the coordinate of
  % the bottom left corner of the axis is available in \x1,
  % and the y-component in \y1
  \p1=(ax.south west),
  % similarly for the top right corner
  \p2=(ax.north east),
  % the value of a calculation can be saved to \n{<something>}
  \n1={0.05*(\y2-\y1)}
  % note that the macros defined above are not available
  % outside this specific path
  in
  % save coordinates for the rectangle
  % (you could also have used \fill let ... (\x1,\y2+\n1) rectangle (\x2,\y2+2*\n1); directly)
  coordinate (lowerleft) at (\x1,\y2+\n1)
  coordinate (upperright) at (\x2,\y2+2*\n1)
;

\fill[green] (lowerleft) rectangle (upperright);

% the anchors of the axis can be used to place stuff
\node at (ax.north) {node1};

% another possibility, again with the calc-library
% ($(ax.south west)!1.15!(ax.north west)$) is using a "partway modifier"
% draw a line from ax.south west to ax.north west, then make it 15% longer
% (multiply length by 1.15), this coordinate is at that point
\fill [blue] ($(ax.south west)!1.15!(ax.north west)$) rectangle 
             ($(ax.south east)!1.2!(ax.north east)$);
\end{tikzpicture}
\end{document} 

Alternative 2

There is the rel axis cs coordinate system, which lets you specify coordinates as fractions of the axis coordinate system. (rel axis cs:0,0) is the bottom left of the axis, (rel axis cs:1,1) is the top right.

rel axis cs can be combined with axis cs coordinates using a perpendicular coordinate specification, see point 1 in TikZ: What EXACTLY does the the |- notation for arrows do?, and example in code below.

\RequirePackage{luatex85}
\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usetikzlibrary{calc}

\newcommand\xmi{0}
\newcommand\xma{3.5}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
  % note addition of name=ax
  name=ax,
  xlabel={x},
  ylabel={y},
  no markers,
  clip mode=individual,
  samples=100,
  xmin=\xmi,
  xmax=\xma,
  domain=\xmi:\xma,
]

\addplot {6*sin(deg(x))};

\node at (2,8) {text};

% you can use relative axis coordinates
\fill [green] (rel axis cs:0,1.05) rectangle (rel axis cs:1,1.1);

% to combine relative and explicit coordinates
\fill [red] ({rel axis cs:0,1.15} -| {axis cs:0.5,0}) % y-component of (rel axis cs:0,1.15), x-component of (axis cs:0.5,0)
            rectangle
            ({rel axis cs:0,1.2} -| {axis cs:2,0});

\fill [blue] ({rel axis cs:0,1.15} -| {axis cs:2.5,0}) rectangle
              ({rel axis cs:0,1.2} -| {axis cs:\xma,0});
\end{axis}
\end{tikzpicture}
\end{document}