[Tex/LaTex] Specify x and y in different coordinate systems in pgfplots similar to gnuplot

Question

In gnuplot you can specify positions in several coordinate systems:
First, Second, Graph, Screen, Character.

What are the equivalents in pgfplots?
I know the following:

• First : axis cs:
• Second : axis cs: (manual suggests using two axis on top of each other)
• Graph : rel axis cs:
• Screen : can be done with the help of barycentric cs:
\begin{axis}[name=s]
\end{axis}
\coordinate (topright) at (s.outer north east);
\coordinate (bottomleft) at (s.outer south west);
\node at (barycentric cs:bottomleft=0.5,topright=0.5) {CENTER};
  (possibly not the most convenient way and only available outside of \begin{axis}\end{axis})

How can one give x and y coordinate in different systems?

Example:

\documentclass{standalone}

\usepackage{pgfplots}
\usepackage{pgfplotstable} % For \pgfplotstableread
\pgfplotsset{compat=1.10}

\pgfplotstableread{
0.01    1.00
0.02    2.00
0.03    3.00
0.04    4.00
0.05    5.00
}\datatable

\begin{document}
\begin{tikzpicture}
\begin{axis}[ymin=0, ymax=6]
\addplot table  {\datatable};
\node at (axis cs:0.02,3) {A};
\node at (rel axis cs:0.66,0.5){B};
% \node at (rel axis cs:0.66, axis cs:1){C}; % HOW CAN I DO THIS?
\end{axis}
\end{tikzpicture}
\end{document}

Update

I figured out that there are many ways to specify coordinates using

• different coordinate systems (default (i.e. without specifying): axis description cs),
• predefined nodes,
• predefined anchors, or
• computed values.

Example using the predefined node current axis:

\usetikzlibrary{calc}
\coordinate (topleft) at ($(current axis.north west) + (-1.3cm,0.3cm)$);

These are described in following chapters of the pgfplots manual:

• 4.9.1 Placement of Axis Descriptions
  coordinate systems: axis description cs, ticklabel cs
• 4.9.2 Alignment of Axis Descriptions
  anchors near ticklabel, near ticklabel opposite
• 4.14.2 Accessing Computed Limit Ranges
  \pgfkeysvalueof{/pgfplots/xmin}
\pgfkeysvalueof{/pgfplots/xmax}
\pgfkeysvalueof{/pgfplots/ymin}
\pgfkeysvalueof{/pgfplots/ymax}
\pgfkeysvalueof{/pgfplots/width}
\pgfkeysvalueof{/pgfplots/height}
• 4.17.1 Accessing Axis Coordinates in Graphical Elements
  coordinate systems axis cs, axis direction cs and rel axis cs
  nodes current plot begin current plot end
• 4.17.2 Placing Nodes on Coordinates of a Plot
  \tikz\pos={<fraction>}

• 4.19 Alignment Options (!)
  node current axis with
  anchors center, outer center, outer north west, north west, left of north west, origin, left of origin, …

• 4.20 The Picture’s Size: Bounding Box and Clipping
  \pgfresetboundingbox, trim

• 4.24 Transforming Coordinate Systems
  coordinate system data cs
• 5.7 Grouping plots
  anchor c<x>r<x>.center/left/right

Relevant sections of the pgf manual:

• 17.5.1 Positioning Nodes Using Anchors – center, north west, north, north east, east, south east, south, south west, west, base, mid
• 17.5.2 Basic Placement Options – above/below/right/left,
above left/right, below left/right
• 17.5.3 Advanced Placement Options – \usetikzlibrary{positioning}

Answer 1

As far as I know you can't address components of different coordinate systems. In fact, I don't think it makes too much sense. However, you can use orthogonal identifiers such as

\documentclass[border=5mm]{standalone}

\usepackage{pgfplots}
\usepackage{pgfplotstable} % For \pgfplotstableread
\pgfplotsset{compat=1.10}

\pgfplotstableread{
0.01    1.00
0.02    2.00
0.03    3.00
0.04    4.00
0.05    5.00
}\datatable

\begin{document}
\begin{tikzpicture}
\begin{axis}[ymin=0, ymax=6]
\addplot table  {\datatable};
\node at (axis cs:0.02,3) {A};
\node at (rel axis cs:0.66,0.5){B};
\node at ({rel axis cs:0.66,0}|-{axis cs:0,1}){C}; %<=== Like this
\end{axis}
\end{tikzpicture}
\end{document}