Binomial tree in pgfplots

Question

Question

Recombining tree solution with pgfplots (not pure TikZ).

Context

There are multiple posts on trees (here, here, here and some more) and solutions are declined in multiple ways (matrix of nodes, trees, etc). However I don't see solutions with pgfplots.

In MWE trying to adapt here,I am stuck with the drawing of arrows in 2 ways.

1. first set of arrows going from left to right
2. second set of arrows going from right to left

    \documentclass{standalone}
    \usepackage{pgfplots}
    \usetikzlibrary{arrows.meta} 

    \begin{document}

    \def\mallevel{5} % a number of levels we wish to get

    \tikzset{
        inner sep=0pt, outer sep=2pt, % some node reserve
        malarrow/.style={->, shorten >=0pt, shorten <=-2pt, -{Stealth[length=5pt, width=3pt,     inset=1pt]},
        }, % a style for arrows, -2 is a shift back (an experiment)
        malnode/.style={text=white,draw=blue!50, minimum width=5mm, circle, inner sep=1pt,font=\tiny,fill=blue,opacity=0.5,text opacity=1}, % a style for nodes
    }
    \begin{tikzpicture}
        \begin{axis}[
                xmin = 0,
                xmax = \mallevel,
                ymin = -\mallevel,
                ymax = \mallevel,
                xlabel = $x$,
                yticklabel=\empty,
                y axis line style={draw=none},  
                clip=false,     
            ]

        \foreach \x in {0,...,\mallevel}
        {
            \foreach \y in {0,...,\x}
            {
                \pgfmathparse{-\x/2+\y} % move up by a half of a tree (vertical direction)
                \let\movey=\pgfmathresult 
                \edef\temp{\noexpand
                    \node[malnode] (\x-\y) at (axis cs:\x,\movey) {{\x}-{\y}};
                }
                \temp

                %%%%%%%% draw the arrows    %%%%%%%%
                \ifnum\x>0 
                    \pgfmathparse{int(\x-1)}
                    \let\previousx=\pgfmathresult % previous level (horizontal direction)
                    \ifnum\y>0 
                        \pgfmathparse{int(\y-1)}
                        \let\previousy=\pgfmathresult % previous level (vertical direction)
    %                       \node[malnode] (\previousx-\previousy) at (axis cs:\previousx,\previousy) {check};
    %                       \draw[malarrow] (\previousx-\previousy) -- (\x-\y);
                        \fi 
                    \fi 
                } 
            } 
            \end{axis}
        \end{tikzpicture}

    \end{document}

Answer 1

Depending on what exactly you're doing, it might be better to do this sort of thing outside the axis. But anyways, you mainly just need to repeat the \edef\temp{...}\temp trick. And using evaluate you can simplify the code a bit, no need for the \pgfmathparse etc.

Don't know if the arrows are how you want them, but you should be able to fix that if not.

\documentclass{standalone}
\usepackage{pgfplots}
\usetikzlibrary{arrows.meta} 

\begin{document}

\def\mallevel{5} % a number of levels we wish to get

\tikzset{
    inner sep=0pt, outer sep=2pt, % some node reserve
    malarrow/.style={->, shorten >=0pt, shorten <=-2pt, -{Stealth[length=5pt, width=3pt,     inset=1pt]},
    }, % a style for arrows, -2 is a shift back (an experiment)
    malnode/.style={text=white,draw=blue!50, minimum width=5mm, circle, inner sep=1pt,font=\tiny,fill=blue,opacity=0.5,text opacity=1}, % a style for nodes
}
\begin{tikzpicture}
    \begin{axis}[
            xmin = 0,
            xmax = \mallevel,
            ymin = -\mallevel,
            ymax = \mallevel,
            xlabel = $x$,
            yticklabel=\empty,
            y axis line style={draw=none},  
            tickwidth=0,
            clip=false,     
        ]

    \foreach \x in {0,...,\mallevel}
    {
        \foreach [evaluate={
                    \movey=-\x/2+\y;
                    \previousy=int(\y-1);
                    \previousx=int(\x-1)}
                    ] \y in {0,...,\x}
        {
            \edef\temp{\noexpand
                \node[malnode] (\x-\y) at (axis cs:\x,\movey) {{\x}-{\y}};
            }
            \temp

            %%%%%%%% draw the arrows    %%%%%%%%
            \ifnum\x>0 
                \ifnum\y>0 
                       \edef\temp{%
                        \noexpand\draw[malarrow] (\previousx-\previousy) -- (\x-\y);
                        \noexpand\draw[malarrow] (\previousx-\previousy) -- (\x-\previousy);
                        }
                        \temp
                    \fi 
                \fi 
            } 
        } 
        \end{axis}
    \end{tikzpicture}

\end{document}