pgfplots，將裝飾箭頭中心放在指定位置

Question

我想在參數曲線的垂直部分畫一個箭頭。例如，考慮以下擺線。

\documentclass{scrbook}

\usepackage{tikz}
\usepackage{pgfplots}

\usetikzlibrary{
    arrows.meta
  , bending
  , decorations.markings
  }
\pgfplotsset{compat = 1.17}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[
        width           = 0.4\textwidth
      , height          = 0.25\textwidth
      , axis equal
      , axis lines      = middle
      , enlargelimits   = false
      , tick style      = {draw = none}
      , ymin            = {0.0}
      , xtick           = \empty
      , ytick           = \empty
      ]
        \addplot+[
            no markers
          , thick
          , domain =  -2.08869:8.37188
          , smooth
          , postaction = {decorate}
          , decoration = {
                markings
              , mark = at position 0.132010 with {\arrow{Stealth[length = 2mm, bend]}}
              , mark = at position 0.210413 with {\arrow{Stealth[length = 2mm, bend]}}
              }
          ] ({x - 1.5 * sin(x r)}, {1.5 - 1.5 * cos(x r)});
    \end{axis}
\end{tikzpicture}

\end{document}

我計算了相應的積分，並知道曲線「垂直」部分的位置分別為 0.132 和 0.210，以總長度的分數表示。結果看起來很難看，因為這是箭頭尖端的位置：

如何將箭頭的中心放在這些點上，而不是其尖端？

還有一個附帶問題：我指定了“彎曲”，但它看起來根本不彎曲。為什麼？

Answer 1

因為您明確定義了標記的長度\arrow。您可以使用decoration tranform，請參閱pgfmanual§24.4.1。在本例中，我將箭頭移動了一半長度。

這是輸出，藍色是你的圖，篩選後的圖是半透明的紅色。

% arara: lwpdflatex
\documentclass{scrbook}

\usepackage{tikz}
\usepackage{pgfplots}

\usetikzlibrary{
    arrows.meta
  , bending
  , decorations.markings
  }
\pgfplotsset{compat = 1.17}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[
            width           = 0.4\textwidth
            , height          = 0.25\textwidth
            , axis equal
            , axis lines      = middle
            , enlargelimits   = false
            , tick style      = {draw = none}
            , ymin            = {0.0}
            , xtick           = \empty
            , ytick           = \empty
        ]
        \addplot+[
            no markers
            , thick
            , domain =  -2.08869:8.37188
            , smooth
            , trig format=rad
            , postaction = {decorate}
            , decoration = {
                    markings
                    , mark = at position 0.132010 with {\arrow{Stealth[length = 2mm, bend]}}
                    , mark = at position 0.210413 with {\arrow{Stealth[length = 2mm, bend]}}
                }
        ] ({x - 1.5 * sin(x)}, {1.5 - 1.5 * cos(x)});
        \addplot+[
            no markers
            , thick,opacity=0.5
            , domain =  -2.08869:8.37188
            , smooth
            , trig format=rad
            , postaction = {decorate}
            , decoration = {
                    markings
                    , transform={xshift=1mm}
                    , mark = at position 0.132010 with {\arrow{Stealth[length = 2mm, bend]}}
                    , mark = at position 0.210413 with {\arrow{Stealth[length = 2mm, bend]}}
                }
        ] ({x - 1.5 * sin(x)}, {1.5 - 1.5 * cos(x)});
    \end{axis}
\end{tikzpicture}

\end{document}