Connect interseption points with arrows in foreach loop in pgfplots

Question

I am looking for a way to draw arrows between each point in a sequence of points calculated in a foreach loop as intersections of vertical lines with a graph inspired from here.

\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{fillbetween}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            xmin=0,
            xmax=6,
            ymin=0,
            ymax=2]
            \addplot[
            name path global=A, 
            ]
            coordinates {
                (0,1)
                (6,1)
             };
        \foreach [evaluate=\i as \n using {5-5/2^(\i/2)}] \i in {1,...,20} {%
            \edef\temp{%
                \noexpand \path[name path=AA-\i] (axis cs:\n,0) -- (axis cs:\n,2);
                \noexpand \path[name intersections={of=A and AA-\i},];
                \noexpand \coordinate (A1-\i)  at (intersection-1);
                \noexpand \draw[fill=blue!50!white, opacity=0.5] (A1-\i) circle[radius=2pt] node (\i) {};
            }\temp
        }
    \end{axis}
\end{tikzpicture}

\end{document}

For the nodes of the arrows I definded the nades as \i in my code and now I 'just' have to conect the 1st and 2nd, 2nd and 3rd, n-1th and nth node.

I already looked here and here, but this is not exactly what I am looking for

score 3 · Answer 1 · answered Apr 03 '21 at 01:16

You do not need to let TikZ find the intersections between known horizontal and vertical lines, and the whole \edef\temp stuff is not needed here either. This draws these arrows, but as you can see as soon as the distance between the circle centers becomes smaller than twice the radius the result be comes a bit uncompelling.

\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{fillbetween}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            xmin=0,
            xmax=6,
            ymin=0,
            ymax=2]
            \addplot[
            name path global=A, 
            ]
            coordinates {
                (0,1)
                (6,1)
             };
        \path foreach [evaluate=\i as \n using {5-5/2^(\i/2)}] \i in {1,...,20} {%
         (\n,1) node[fill=blue!50!white, opacity=0.5,circle,inner
            sep=2pt,draw] (\i) {}
         \ifnum\i&gt;1
          (\the\numexpr\i-1) edge[-stealth] (\i)
         \fi
         }; 
    \end{axis}
\end{tikzpicture}

\end{document}

Thanks for your answer. Is your example also suitable to draw the arrows on a non-horizontal line? — Excelsior, Apr 03 '21 at 08:54

score 1 · Accepted Answer · answered Apr 03 '21 at 09:30

For a non-horizontal line, where it is easiest to take the points from the intersections rather than compute them directly, here's some code that works. I've simplified the original code slightly by defining nodes straight-away rather than coordinates and then drawing the circles. Then I've separated out the arrow-drawing code into a separate loop because I agree with the answer from user238301 that once the points get close together then the arrows don't look great so this makes it easy to select which nodes get connected.

\documentclass[border=5pt]{standalone}
%\url{https://tex.stackexchange.com/q/591212/86}
\usepackage{pgfplots}
\pgfplotsset{compat=newest} 
\usetikzlibrary{intersections,arrows.meta}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            xmin=0,
            xmax=6,
            ymin=0,
            ymax=2]
            \addplot[
            name path global=A, 
            ]
            coordinates {
                (0,1)
                (6,1)
             };
        \foreach [evaluate=\i as \n using {5-5/2^(\i/2)}] \i in {1,...,20} {%
            \edef\temp{%
                \noexpand \path[name path=AA-\i] (axis cs:\n,0) -- (axis cs:\n,2);
                \noexpand \path[name intersections={of=A and AA-\i},] node[fill=blue!50!white,opacity=0.5,draw,inner sep=2pt,circle] (A1-\i)  at (intersection-1) {};
            }\temp
        }
        \foreach[evaluate=\i as \prev using int(\i-1)] \i in {2,...,5} {
        \edef\temp{\noexpand\draw[-Stealth] (A1-\prev) -- (A1-\i);}\temp
        }
    \end{axis}
\end{tikzpicture}

\end{document}

Thanks for this general solution. I was not aware that the different intersections are somehow saved for futher processing. — Excelsior, Apr 05 '21 at 16:23

Connect interseption points with arrows in foreach loop in pgfplots

2 Answers2