为什么我的 TikZ 线不直？

Question

这个 tikz 图片代码：

\documentclass[tikz]{standalone}
\usepackage{tikz}

\begin{document}
    \begin{tikzpicture}
        \coordinate (a) at (0, 1);
        \node at (a) {\textbullet};
        \node[above left] at (a) {$\mathbf{a}$};

        \coordinate (b) at (5, 5);
        \node at (b) {\textbullet};
        \node[above right] at (b) {$\mathbf{b}$};

        \draw (a) -- (b);

        \coordinate (c) at (-1, 0);
        \node at (c) {\textbullet};
        \node[above left] at (c) {$\mathbf{c}$};
        \draw[dashed] (c) -- (b);
    \end{tikzpicture}
\end{document}

产生以下输出：

为什么这两条线不重叠（这是浮点错误造成的吗？），我该如何修复它？

Answer 1

正如您在上面的评论中所看到的，您的问题与几何无关TikZ，而是与几何有关（当然，如果我正确理解了您的问题的话）。证明：参见下面的代码并获得结果。

\documentclass[tikz,border=2mm]{standalone}
    \usetikzlibrary{backgrounds}

\begin{document}
    \begin{tikzpicture}
\draw[help lines] (-1.5,-0.5) grid (5.5,5.5);
        \coordinate (a) at (0, 1);
        \node at (a) {\textbullet};
        \node[above left] at (a) {$\mathbf{a}$};

        \coordinate (b) at (5, 5);
        \node at (b) {\textbullet};
        \node[above right] at (b) {$\mathbf{b}$};

        \draw (a) -- (b);

        \coordinate (c) at (-1, 0);
        \node at (c) {\textbullet};
        \node[below right] at (c) {$\mathbf{c}$};
        \draw[dashed] (c) -- (b);
% the right position for coordinate c, 
% that it to be lying on strait through b and c is (-5/4,0)
        \coordinate (d) at (-5/4, 0);
        \node at (d) {\textbullet};
        \node[below left] at (d) {$\mathbf{c'}$};
    \scoped[on background layer]
        \draw[dashed,red!30, ultra thick] (d) -- (b);
    \end{tikzpicture}
\end{document}