The objective is to have arrows along a path for every 1-unit step we take horizontally or vertically. The arrows would be aligned midway between major grid tick marks, e.g. midway along (0,0)--(0,1). To give context, these arrows could be used to show east-west and north-west movement along a rectangular grid.
I have managed to handle the case where the x and y units are the same. Below I have tweaked the parameters to have the blue arrows aligned at the midpoint (grid minor line colored in cyan). But I failed with the red arrows. The problem is that the x-scale and y-scale are quite different. If the step is tweaked to match horizontal distances (parallel to the x-axis), then it will not match the vertical distances. How to fix this?
To clarify, if I give a path like (0,0)--(10,0), I would like an arrow midway along (0,0)--(1,0), then midway along (1,0)--(2,0), and so on. Otherwise I would just use something like [midway](0,0)--(10,0) or [pos=0.5](0,0)--(10,0). In my code I have to tweak the "starting value" as well as the "step". It would obviously be much nicer to have a solution where I could set a starting value at 0.5 (meaning at the midway point between two major grid tick lines) and a step equal to 1 (meaning moving by 1 unit of image scale --- not units of the axis --- along both x and y directions).
\documentclass[border=3pt,tikz]{standalone} \usepackage{pgfplots} \pgfplotsset{compat=newest} \usetikzlibrary{calc} % Required for coordinate calculations \usetikzlibrary{decorations.markings} % Required for decorations along path % Define a TikZ style for arrows on path: color as optional argument % \usetikzlibrary{decorations.markings, arrows.meta} \tikzset{ arrows along path/.style args={#1/#2/#3}{% >=latex, decoration={ markings, mark=between positions #1 and #2 step #3 with { \arrow{latex} } }, postaction={decorate} }, arrows along path/.default={0/1/0.5} } % x has range 5 % y has range 3000 % 3000/5 = 600 % 5/3000 = 0.00166666666 \begin{document} \begin{tikzpicture} \begin{axis}[ x = 2cm, y = 0.0033cm,% tweaked to match y/x=5/3000 ratio xmin=0, xmax=5, ymin=500, ymax=3500, xtick = {0,1,...,5}, ytick = {500,1000,...,3500}, tick align=outside, minor tick num=1, minor tick length=4pt, minor tick style={color=cyan,thick}, minor grid style={color=cyan,thick}, grid = both, clip = false, ] \coordinate (A) at (0,2000); \coordinate (B) at (5,2000); \coordinate (C) at (0,1000); \coordinate (D) at (1,1000); \coordinate (E) at (1,3000); \coordinate (F) at (4,3000); \coordinate (G) at (4,1000); \coordinate (H) at (5,1000); % Blue line \draw [blue, thick, arrows along path={0.118/1/0.20}] (A) -- (B); % Red line \draw [red, thick, arrows along path={0.051/1/0.079}] (C) -- (D) -- (E) -- (F) -- (G) -- (H); \end{axis} \end{tikzpicture} \end{document} 
Second implementation, also fails to get the correct output:
\documentclass[border=3pt,tikz]{standalone} \usepackage{pgfplots} \pgfplotsset{compat=newest} \usetikzlibrary{calc} % Required for coordinate calculations \usetikzlibrary{decorations.markings} % Required for decorations along path % Define a TikZ style for arrows on path: color as optional argument % \usetikzlibrary{decorations.markings, arrows.meta} \tikzset{ arrows along path/.style args={#1/#2/#3}{% >=latex, decoration={ markings, mark=between positions #1 and #2 step #3 with { \arrow{latex} } }, postaction={decorate} }, arrows along path/.default={0/1/0.5} } \begin{document} \begin{tikzpicture} \begin{axis}[ x=2cm, y=0.0033cm, xmin=0, xmax=5, ymin=500, ymax=3500, xtick = {0,1,...,5}, ytick = {500,1000,...,3500}, tick align=outside, minor tick num=1, minor tick length=4pt, minor tick style={color=cyan,thick}, minor grid style={color=cyan,thick}, grid = both, clip = false, ] \coordinate (A) at (0,2000); \coordinate (B) at (5,2000); \coordinate (C) at (0,1000); \coordinate (D) at (1,1000); \coordinate (E) at (1,3000); \coordinate (F) at (4,3000); \coordinate (G) at (4,1000); \coordinate (H) at (5,1000); % Blue line \draw [blue, thick, arrows along path={0.12/1/0.20}] (A) -- (B); % Red line \pgfplotsextra{ \foreach \P/\Q in {C/D, D/E, E/F, F/G, G/H}{ \draw [red, thick, arrows along path={0.6/1/0.40}] (\P) -- (\Q); } } \end{axis} \end{tikzpicture} \end{document} 
A variant with a node and a dot, that sort of look like the ideal implementation for its simplicity, but fails just the same.
\documentclass[border=3pt,tikz]{standalone} \usepackage{pgfplots} \pgfplotsset{compat=newest} \usetikzlibrary{calc} % Required for coordinate calculations \usetikzlibrary{decorations.markings} % Required for decorations along path % set up all axes \pgfplotsset{% every axis/.append style={% axis styles %ticks = none, grid = both, clip = false,% remove clip }% }% % Define a TikZ style for arrows \tikzset{ marks along path/.style args={#1/#2/#3/#4}{% decoration={ markings, mark=between positions #1 and #2 step #3 with { \node[#4] at (0,0) {}; } }, postaction={decorate} }, marks along path/.default={0/1/0.5/shape=circle,fill=black,draw=black,inner sep=0pt,minimum size=4mm,line width=2pt} } \begin{document} \begin{tikzpicture} \begin{axis}[ x = 2cm, y = 0.0033cm,% tweaked to match y/x=5/3000 ratio xmin=0, xmax=5, ymin=500, ymax=3500, xtick = {0,1,...,5}, ytick = {500,1000,...,3500}, tick align=outside, minor tick num=1, minor tick length=4pt, minor tick style={color=cyan,thick}, minor grid style={color=cyan,thick}, grid = both, clip = false, ] \coordinate (A) at (0,2000); \coordinate (B) at (5,2000); \coordinate (C) at (0,1000); \coordinate (D) at (1,1000); \coordinate (E) at (1,3000); \coordinate (F) at (4,3000); \coordinate (G) at (4,1000); \coordinate (H) at (5,1000); % Blue line \draw[blue, line width=2pt, marks along path={0/1/0.5/shape=circle,fill=blue,draw=black}] (A) -- (B); \draw[red, line width=2pt, marks along path={0/1/0.5/shape=circle,fill=red,draw=black}] (C) -- (D) -- (E) -- (F) -- (G) -- (H); \end{axis} \end{tikzpicture} \end{document} 
