使坡度场中的所有线长度相同

Question

在以下斜率场中，如何使所有线具有相同的长度？

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{tikzpicture}
            \begin{axis}[
                scale = 0.85,
                axis lines = middle,
                xlabel=$x$,
                ylabel=$M(t)$,
                enlargelimits,
                xtick = {15},
                ytick = {70},
                xmin = -0.5, xmax = 20.5,
                ymin = -0.5, ymax = 70.5,
                zmin = 0, zmax = 1,
                view = {0}{90},
                ]
                
                % Define the differential equation
                \def\DE{(40 - y) / 4}
                
                % Draw the slope field
                \addplot3[black!75, samples=15,
                quiver = {
                    u = {1/sqrt(1+(\DE)^2)},
                    v = {(\DE)/sqrt(1+(\DE)^2)},
                    scale arrows = 0.5,
                },
                domain = 0:20,
                domain y = 0:70,
                ] {0};
            \end{axis}
        \end{tikzpicture}
\end{document}