TikZ position of an imported image

Question

I have a given diagram of measured values:

I wish to draw on above diagram. MWE:

\documentclass{scrartcl}
\usepackage{tikz}

\begin{document}

  \begin{tikzpicture}

    \node at (0, 0) {\resizebox{\linewidth}{!}{\includegraphics{diagram}}};
    \draw
      (0, 0) -- (2, 2);

  \end{tikzpicture}

\end{document}

The coordinates of the \node and the \draw command are each (0, 0), but in the output the drawn ((0, 0) -- (2, 2)) line begins somewhere in the area:

What do I wrong and why it's happens? What is to do for equivalent coordinates on both plain, to make intuitive drawing on the diagram. All possible solutions are welcome, also a complete redraw of the diagram too.

Thank you for your help and effort in advance!

Answer 1

I often use TikZ to draw paths above another picture, say a photograph. For this I use

\node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics{somepicture.jpg}};
\begin{scope}[x={(image.south east)},y={(image.north west)}]
% draw your things here
\end{scope}

It creates a node called 'image' and with the scope command, the image's lower left corner gets coordinates (0,0) while the upper right corner is (1,1). So you can use coordinates relative to the image to place your nodes etc.

If you need to find coordinates of certain points you want to use in your picture, you can use

\draw[help lines, very thin, step=0.02] (0,0) grid (1,1);
\draw[help lines,thin,xstep=.1,ystep=.1] (0,0) grid (1,1);
\foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
\foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }

this will draw a grid with coordinate labels on top of your image, which allows you to figure out the proper coordinates. When you are done, just remove the grid.

Here is an example with the grid:

\begin{tikzpicture}
  \node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[width=5cm]{qOr3L.png}};
  \begin{scope}[x={(image.south east)},y={(image.north west)}]
  \draw[help lines, very thin, step=0.02] (0,0) grid (1,1);
  \draw[help lines,thin,xstep=.1,ystep=.1] (0,0) grid (1,1);
  \foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
  \foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
  \draw[<-] (0.45,0.4) -- (1.1,1.1) node[above] {marmot's crystal ball};
  \end{scope}
\end{tikzpicture}

Produces:

I use these commands to put labels on different things in photographs.

Answer 2

One simpleminded proposal is to draw the full thing with TikZ/pgfplots. I reproduced your plot using my crystal ball. The advantage is that this will install automatically the appropriate axis coordinate system for you, such that you can add whatever lines to your plot (as long as you are inside the axis).

\documentclass{scrartcl}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}
\begin{axis}[font=\sffamily,
grid=major,
xmin=0,
xmax=3500,
ymin=0,
ymax=0.2,
grid=both,
major grid style={draw=gray!60},
minor grid style={densely dotted},
minor x tick num=4,
minor y tick num=1,
xtick={0,500,...,3500},
ytick={0,0.02,...,0.2},
yticklabel style={/pgf/number format/fixed},
width=\linewidth,
ylabel={Messgr\"osse $y$},
xlabel={Zeit $t$ [$s$]},
title={Sprungantwort der Strecke}
]
\addplot[domain=100:3500,samples=201,smooth] {0.2*(1-exp(-(x-100)/606))};
\draw[red] (0,0) -- (1000,0.04);
\end{axis}
\end{tikzpicture}
\end{document}

Answer 3

A manual hack, together with use of anchor is as follows:

\documentclass{scrartcl}
\usepackage{tikz}

\begin{document}

  \begin{tikzpicture}[xscale=0.03884,yscale=53.44]
    \node[anchor=south west,inner sep=0] at (-29.53,-0.0156) {\resizebox{\linewidth}{!}{\includegraphics{diagram}}};
    % A few red lines and dots to show it lining up:
    \draw[red] (0, 0.1) -- (20, 0.1);
    \draw[red] (0,0) -- (0,0.1);
    \node at (0,0) {\color{red}$\cdot$};
    \node at (10,0.2) {\color{red}$\cdot$};
    \node at (100,0.1) {\color{red}$\cdot$};
  \end{tikzpicture}

\end{document}

Note that I divided the x coordinates by 10 to avoid LaTeX getting unhappy with large dimensions, so (100,0.1) really corresponds to the point (1000,0.1) on the plot. Additionally, if you put this in a document with (e.g.) different page dimensions, you'd probably need to adjust xscale, yscale, and the coordinates of the node all over again...