[Tex/LaTex] Drawing a semicircle in 3D with pgfplots

pgfplots

I'm trying to make a plot of a torus with a semicircle drawn on its surface. The following MWE

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[hide axis,
        axis equal,
        view={-37.5}{30}]
            \addplot3[surf,
            shader=interp,
            samples=20,
            domain=0:2*pi,y domain=-pi:pi,
            z buffer=sort]
            ({(1+0.25*cos(deg(x)))*cos(deg(y))}, 
             {(1+0.25*cos(deg(x)))*sin(deg(y))}, 
             {0.25*sin(deg(x))});

            \addplot3[color=black,
            samples=20,
            domain=0:pi,
            line width=2.0pt]
            ({cos(deg(x))}, 
             {sin(deg(x))}, 
             {0.25});
    \end{axis}
\end{tikzpicture}
\end{document}

generates

torus

which shows the problem: the semicircle gets closed. I'm not particularly well-versed in pgfplots (most of the above is stuff I picked up from examples and plots generated with the Matlab script matlab2tikz). Is there an easy way to get the unclosed semicircle I'm after?

Just did a quick test: the following code

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[hide axis,
        axis equal]
            \addplot[color=black,
            samples=20,
            domain=0:pi,
            line width=2.0pt]
            ({cos(deg(x))}, 
             {sin(deg(x))});
    \end{axis}
\end{tikzpicture}
\end{document}

shows that this problem doesn't occur in a 2D plot:

semicircle2D

Best Answer

You can fix this by using

 \addplot3[color=black,
        samples=20,
        domain=0:pi,
        line width=2.0pt,samples y=0]

so that the addplot3 command only plots a curve, not a surface.

screenshot

Further details can be found in Section 4.6.9: Parameterized plots of the pgfplots documentation.

Related Solutions

PGFPlots – How to Plot Two Time Series with Bounds Using PGFPlots

This happens because PGFPlots only uses one "stack" per axis: You're stacking the second confidence interval on top of the first. The easiest way to fix this is probably to use the approach described in "Is there an easy way of using line thickness as error indicator in a plot?": After plotting the first confidence interval, stack the upper bound on top again, using stack dir=minus. That way, the stack will be reset to zero, and you can draw the second confidence interval in the same fashion as the first:

\documentclass{standalone}
\usepackage{pgfplots, tikz}

\usepackage{pgfplotstable}

\pgfplotstableread{
temps   y_h y_h__inf    y_h__sup    y_f y_f__inf    y_f__sup    

1   0.237340    0.135170    0.339511    0.237653    0.135482    0.339823    
2   0.561320    0.422007    0.700633    0.165871    0.026558    0.305184    
3   0.694760    0.534205    0.855314    0.074856    -0.085698   0.235411    
4   0.728306    0.560179    0.896432    0.003361    -0.164765   0.171487    
5   0.711710    0.544944    0.878477    -0.044582   -0.211349   0.122184    
6   0.671241    0.511191    0.831291    -0.073347   -0.233397   0.086703    
7   0.621177    0.471219    0.771135    -0.088418   -0.238376   0.061540    
8   0.569354    0.431826    0.706882    -0.094382   -0.231910   0.043146    
9   0.519973    0.396571    0.643376    -0.094619   -0.218022   0.028783    
10  0.475121    0.366990    0.583251    -0.091467   -0.199598   0.016664    
}{\table}

\begin{document}
\begin{tikzpicture}

    \begin{axis}
    % y_h confidence interval
    \addplot [stack plots=y, fill=none, draw=none, forget plot]   table [x=temps, y=y_h__inf]   {\table} \closedcycle;
    \addplot [stack plots=y, fill=gray!50, opacity=0.4, draw opacity=0, area legend]   table [x=temps, y expr=\thisrow{y_h__sup}-\thisrow{y_h__inf}]   {\table} \closedcycle;
    % subtract the upper bound so our stack is back at zero
    \addplot [stack plots=y, stack dir=minus, forget plot, draw=none] table [x=temps, y=y_h__sup] {\table};

    % y_f confidence interval
    \addplot [stack plots=y, fill=none, draw=none, forget plot]   table [x=temps, y=y_f__inf]   {\table} \closedcycle;
    \addplot [stack plots=y, fill=gray!50, opacity=0.4, draw opacity=0, area legend]   table [x=temps, y expr=\thisrow{y_f__sup}-\thisrow{y_f__inf}]   {\table} \closedcycle;

    % the line plots (y_h and y_f)    
    \addplot [stack plots=false, very thick,smooth,blue]  table [x=temps, y=y_h]   {\table};
    \addplot [stack plots=false, very thick,smooth,blue]  table [x=temps, y=y_f]   {\table};
    \end{axis}

\end{tikzpicture}
\end{document}

[Tex/LaTex] Drawing piecewise linear plots with pgfplots

Two ways: first, definig a function and plotting each piece separately, as was suggested in What is the clearest way to graph a piecewise function?; second , using two plots (the first approach might seem an overkill here):

\documentclass{article}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}[
  declare function={
    func(\x) = (\x<0.5) * (1)  +  and(\x>0.5, \x<1) * (0);
  }
]
\begin{axis}[
  axis lines=middle,
  enlargelimits
]
\foreach \start/\end  in {0/0.5, 0.5/1} 
  \addplot[domain=\start:\end,blue,thick,samples=10] {func(x)};
\draw[blue,fill=blue] 
  (axis cs:0.5,1) circle (2pt);
\draw[blue,fill=white] 
  (axis cs:0.5,0) circle (2pt);
\end{axis}
\end{tikzpicture} 

\begin{tikzpicture}
\begin{axis}[
  axis lines=middle,
  enlargelimits
]
\foreach \start/\end/\Func  in {0/0.5/1, 0.5/1/0} 
  \addplot[domain=\start:\end,blue,thick,samples=10] {\Func};
\draw[blue,fill=blue] 
  (axis cs:0.5,1) circle (2pt);
\draw[blue,fill=white] 
  (axis cs:0.5,0) circle (2pt);
\end{axis}
\end{tikzpicture} 

\end{document}

enter image description here

The filled and "empty" dots can be produced using small circles placed at the desired location using the axis cs coordinate system. If you want to see the axis beneath the empty dot, add axis on top:

\documentclass{article}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}[
  declare function={
    func(\x) = (\x<0.5) * (1)  +  and(\x>0.5, \x<1) * (0);
  }
]
\begin{axis}[
  axis lines=middle,
  enlargelimits,
  axis on top
]
\foreach \start/\end  in {0/0.5, 0.5/1} 
  \addplot[domain=\start:\end,blue,thick,samples=10] {func(x)};
\draw[blue,fill=blue] 
  (axis cs:0.5,1) circle (2pt);
\draw[blue,fill=white] 
  (axis cs:0.5,0) circle (2pt);
\end{axis}
\end{tikzpicture} 

\begin{tikzpicture}
\begin{axis}[
  axis lines=middle,
  enlargelimits,
  axis on top
]
\foreach \start/\end/\Func  in {0/0.5/1, 0.5/1/0} 
  \addplot[domain=\start:\end,blue,thick,samples=10] {\Func};
\draw[blue,fill=blue] 
  (axis cs:0.5,1) circle (2pt);
\draw[blue,fill=white] 
  (axis cs:0.5,0) circle (2pt);
\end{axis}
\end{tikzpicture} 

\end{document}

enter image description here

Best Answer

Related Solutions

PGFPlots – How to Plot Two Time Series with Bounds Using PGFPlots

[Tex/LaTex] Drawing piecewise linear plots with pgfplots

Related Question