Turning Concentric ovals into concentric circles with rectangular patch

Question

I would like to draw the blend of two diagrams. Here is my code for the origin:

\documentclass{article}

\usepackage{pgfplots}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{tkz-euclide}
%\pgfplotsset{compat=newest} %<------ Here
\pgfplotsset{compat=1.11} %<------ Or use this one

\begin{document}

\begin{center}
\begin{tikzpicture}[scale=1.5][x=1cm,y=1cm]
\begin{axis}[xtick={-5,-4,-3,-2,-1,0,1,2,3,4,5}, ytick={-5,-4,-3,-2,-1,0,1,2,3,4,5},
xmin=-5, xmax=5, ymin=-5, ymax=5, grid=major,
xlabel={Galactic longitude $\ell$ in degree},
ylabel={Galactic latitude $b$ in degree},
axis lines = left,
axis line style={->}]

 % rectangle
 \draw[blue] (0,0) circle[radius=1];
 \draw[blue] (0,0) circle[radius=2];
 \draw[blue] (0,0) circle[radius=3];
 \draw[blue] (0,0) circle[radius=4];
 \draw[blue] (0,0) circle[radius=5];
\end{axis}
\end{tikzpicture}
\end{center}
\end{document}

I am trying to draw concentric circles but ended up with ovals. I would like to know how this can be fixed?

Here is the second issue, I would like to put a rectangular patch following the image:

.

I am struggling with these two: turning ovals into circles and placing a rectangular patch on it.

Answer 1

You can use the following:

\documentclass[tikz, border=2mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

\begin{document}

\begin{tikzpicture}[scale=1.5]
  \begin{axis}[
    xmin=-5, xmax=5, ymin=-5, ymax=5, grid=major,
    xtick={-5,...,5}, ytick={-5,...,5},
    unit vector ratio = 1 1,
    xlabel={Galactic longitude $\ell$ in degree},
    ylabel={Galactic latitude $b$ in degree},
    axis lines = left,
    axis line style={->},
    clip=false]

   \pgfplotsinvokeforeach {1,...,5} {
     \draw[blue] (0,0) circle[radius=#1];
   }
   \draw[green, thick, fill=white] (-5,-0.5) rectangle (5,0.5);
  \end{axis}
\end{tikzpicture}

\end{document}

Notes:

• don't pass several optional arguments to tikzpicture (\begin{tikzpicture}[scale=1.5][x=1cm,y=1cm] is incorrect, better use something like \begin{tikzpicture}[scale=1.5, x=1cm,y=1cm]);

• pgfplots compatibility level 1.11 is a bit old, I bumped it to 1.16. \pgfplotsset{compat=newest} may be fine when you are actively working on a plot and are going to fix all problems, but then better hardcode a version to ensure that your plot doesn't silently produce a different result in the future when you recompile it.
  In essence, saying \pgfplotsset{compat=1.16} means: “I have verified that this plot works fine with pgfplots compatibility level 1.16, I want to have the same behavior every time I recompile. Only when I'm ready to fix potential problems introduced by changes in pgfplots will I switch to a newer compatibility level (when more recent versions of pgfplots come out—1.16 is the latest at this point in time)”.

• I used a \pgfplotsinvokeforeach loop to draw the circles;

• I drew the rectangle using the implicit axis cs coordinate system (implicit since compatibility level 1.11);

• I drew it after the circles to make sure its filling covers the circles and grid;

• I used option unit vector ratio = 1 1 to make sure the units on each axis are represented with the same length (this is what makes the circles actual circles as opposed to ellipses).

• clip=false ensures that the left and right sides of the green rectangle are not trimmed (half of the line width would be trimmed otherwise—try it).

Another coordinate system could be used to draw the rectangle patch, this is rel axis cs (see Coordinate system rel axis cs in the pgfplots manual). In this case, the coordinates wouldn't correspond to your data values; instead, the [0,1] range would be mapped to the ranges covered by each of the plot axes (x and y here, but this also works with 3D plots). In other words, the green rectangle could also be drawn this way:

\draw[green, thick, fill=white]
  (rel axis cs:0,0.45) rectangle (rel axis cs:1,0.55);