tikz：不太精确的图形

Question

您是否发现该点(0.78951,1.38506)比前一个点略低，而它应该位于其上方？

\documentclass{standalone}

    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage{amsmath,amssymb,amsfonts,systeme} % Typical maths resource packages
    \usepackage{color}                    % For creating coloured text and background
    \usepackage{array}
    \usepackage{tikz}
    \usepackage{tkz-euclide}
    \usetkzobj{all}
    \usetikzlibrary{arrows.meta}
    \usepackage{siunitx}
    \usepackage{pgfplots}
    \usepackage{filecontents}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[
  grid, grid style=dashed,
  xticklabel style={rotate=90},
  ymin=1.33,ymax=1.39,
  extra y ticks={1.33,1.35,1.37},            % <---
  xmax=1,xmin=0,
  extra x ticks={0},            % <---
  xtick={0,0.20594,0.38019,0.62664,0.78951,1},
  minor tick num=1,
  axis lines = middle,
  set layers,                   % <---
  %axis on top,                  % <---
  xlabel=$x_1$,ylabel=$n_D$,
  x label style={at={(1,0)},right},
  y label style={at={(0,1)},above},
  x tick label style={/pgf/number format/.cd,
                      fixed relative,
                      /pgf/number format/precision=5
                     },
  style={thick}
               ]
  \addplot+[color=red,mark=*,mark options={draw=red,fill=green},thick,smooth]
    coordinates {
    (0.00000, 1.33300)(0.20594,1.37003)(0.38019,1.37944)(0.62664,1.38502)(0.78951,1.38506)(1.00000,1.38511)
    };
  \end{axis}
\end{tikzpicture}
\end{document}

是只有我一个人这样，还是真的这样？在这种情况下，你有什么建议吗？

先感谢您

Answer 1

这是视觉效果，因为红色曲线在点 (0.65664, 1.38502) 处的斜率相对于点 (0.65664, 1.38502) 和 (0.78951,1.38506) 之间的 (实际) 斜率太大。您可以在此放大的屏幕截图 (带有蓝色水平线) 中看到，点 (0.78951,1.38506) 实际上高于前一个点。

\pgfplotsset{compat=1.16}请注意代码中添加的这一行：

\documentclass{standalone}

    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage{amsmath,amssymb,amsfonts,systeme} % Typical maths resource packages
    \usepackage{color}                    % For creating coloured text and background
    \usepackage{array}
    \usepackage{tikz}
    \usepackage{tkz-euclide}
    \usetkzobj{all}
    \usetikzlibrary{arrows.meta}
    \usepackage{siunitx}
    \usepackage{pgfplots}
    \usepackage{filecontents}
    \pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[
  grid, grid style=dashed,
  xticklabel style={rotate=90},
  ymin=1.33,ymax=1.39,
  extra y ticks={1.33,1.35,1.37},            % <---
  xmax=1,xmin=0,
  extra x ticks={0},            % <---
  xtick={0,0.20594,0.38019,0.62664,0.78951,1},
  minor tick num=1,
  axis lines = middle,
  set layers,                   % <---
  %axis on top,                  % <---
  xlabel=$x_1$,ylabel=$n_D$,
  x label style={at={(1,0)},right},
  y label style={at={(0,1)},above},
  x tick label style={/pgf/number format/.cd,
                      fixed relative,
                      /pgf/number format/precision=5
                     },
  style={thick}
               ]
  \addplot+[color=red,mark=*,mark options={draw=red,fill=green},thick,smooth]
    coordinates {
    (0.00000, 1.33300)(0.20594,1.37003)(0.38019,1.37944)(0.62664,1.38502)(0.78951,1.38506)(1.00000,1.38511)
    };
    \draw[ultra thin, blue] (0.62,1.38410)--(0.8,1.38410) ;
    \draw[ultra thin, violet] (0.62664,1.38502) -- (0.70,1.38570);
  \end{axis}

\end{tikzpicture}
\end{document}

tension您可以通过减少（默认情况下为 0.55）来 减轻这种影响：\addplot+[color=red,mark=*,mark options={draw=red,fill=green},thick,smooth,tension=0.25]

给：

Answer 2

0.79 处的点实际上并不低。相反，插值图在 0.62 处的点之后继续增加，这使得它看好像 0.79 的点较低。您可以通过放大图表并仅查看这两个点来看到这种情况。下图显示右侧 0.79 的点确实略高于 0.62 的点：

[这确实是一个扩展评论，只不过你不能将图片放入评论中……]