# Why is there a trace line in this tikz graph?

I am trying to generate a shaded area between three curves: two are y=±√x and third one is y=5.

\documentclass[10pt,multi=False,border=5pt,tikz,class=scrartcl]{standalone}
\usepackage{xcolor}

\usepackage{pgfplots}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{patterns}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\begin{axis}[grid=both, axis equal=false, ymin=-1, ymax=6, xmin=-30, xmax=30,
minor tick num=1,axis lines = middle,
label style={font=\small,at={(ticklabel cs:1.1)}},
tick label style={font=\footnotesize}]
\addplot [thick,dashed,name path=p3] coordinates {(-25,5) (25,5)};
\path [name path=left, intersection segments={of=p1 and p3,sequence=L2 -- R1}];
\path [name path=right, intersection segments={of=p2 and p3,sequence=L1 -- R2}];
\addplot [pattern=north east lines,pattern color=blue,opacity=.8] fill between [
of=left and right,reverse=false];
\addplot+ [only marks,mark=*] coordinates { (-25,5) (25,5)}
node [pos=0,above right] {\footnotesize\textcolor{blue}{$B=(-25,5)$}}
node [pos=1,above left] {\footnotesize\textcolor{blue}{$A=(25,5)$}};
\node [right] at (11,3) {$r=x=y^2$};
\end{axis}
\end{tikzpicture}
\end{document}


• Welcome to TeX.SE! With that nick name you will feel at home here! – CarLaTeX Dec 1 '18 at 14:52
• Please provide a complete MWE (with preamble and all of that stuff) that can be compiled by others. This increases the chance of people offering their help. – Raven Dec 1 '18 at 14:58
• @marmot, OP is asking about the dotted thin line connecting the cuspid and B point. It is very light. – Sigur Dec 1 '18 at 15:09
• also notice how the seemingly dotted line fades away from y=0 to y=5. It is my conjecture that it is not a dotted line but rather a reminiscent of rounding-off error while computing the inverse or the intersections. I'll wait for expert opinions on it. But it would also be helpful if others can recreate and confirm the issue. – rubber duck Dec 1 '18 at 15:16

Big thanks to Sigur for explaining the question to me! The issue is that the paths do not have the appropriate orientation. So I had to reorder the sequences and reverse one to get

\documentclass[10pt,multi=False,border=5pt,tikz,class=scrartcl]{standalone}
\usepackage{xcolor}

\usepackage{pgfplots}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{patterns}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\begin{axis}[grid=both, axis equal=false, ymin=-1, ymax=6, xmin=-30, xmax=30,
minor tick num=1,axis lines = middle,
label style={font=\small,at={(ticklabel cs:1.1)}},
tick label style={font=\footnotesize}]
\addplot [thick,dashed,name path=p3] coordinates {(-25,5) (25,5)};
\path [name path=left,  intersection segments={of=p1 and p3,sequence={R1[reverse] -- L2}}];
\path [name path=right,
intersection segments={of=p2 and p3,sequence={L1 -- R2}}];
\addplot [pattern=north east lines,pattern color=blue,opacity=.8] fill between [
of=left and right,reverse=false];
\addplot+ [only marks,mark=*] coordinates { (-25,5) (25,5)}
node [pos=0,above right] {\footnotesize\textcolor{blue}{$B=(-25,5)$}}
node [pos=1,above left] {\footnotesize\textcolor{blue}{$A=(25,5)$}};
\node [right] at (11,3) {$r=x=y^2$};
\end{axis}
\end{tikzpicture}
\end{document}


How can one debug this? Just draw these paths with arrows.

\documentclass[10pt,multi=False,border=5pt,tikz,class=scrartcl]{standalone}
\usepackage{xcolor}

\usepackage{pgfplots}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{patterns}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\begin{axis}[grid=both, axis equal=false, ymin=-1, ymax=6, xmin=-30, xmax=30,
minor tick num=1,axis lines = middle,
label style={font=\small,at={(ticklabel cs:1.1)}},
tick label style={font=\footnotesize}]
\addplot [thick,dashed,name path=p3] coordinates {(-25,5) (25,5)};
\path [name path=left, thick,draw=blue,-latex,
intersection segments={of=p1 and p3,sequence=L2 -- R1}];
\path [name path=right, thick,draw=red,-latex,
intersection segments={of=p2 and p3,sequence=L1 -- R2}];
\addplot [pattern=north east lines,pattern color=blue,opacity=.8] fill between [
of=left and right,reverse=false];
\addplot+ [only marks,mark=*] coordinates { (-25,5) (25,5)}
node [pos=0,above right] {\footnotesize\textcolor{blue}{$B=(-25,5)$}}
node [pos=1,above left] {\footnotesize\textcolor{blue}{$A=(25,5)$}};
\node [right] at (11,3) {$r=x=y^2$};
\end{axis}
\end{tikzpicture}
\end{document}


This reveals that the left path is a loop, which explains the faint dots on the left. It is a loop because you run through the curved part from top left to bottom right and then through the horizontal part.

This also shows that the horizontal curve is not at all necessary here, for the fills, you could just do

\documentclass[10pt,multi=False,border=5pt,tikz,class=scrartcl]{standalone}
\usepackage{xcolor}

\usepackage{pgfplots}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{patterns}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\begin{axis}[grid=both, axis equal=false, ymin=-1, ymax=6, xmin=-30, xmax=30,
minor tick num=1,axis lines = middle,
label style={font=\small,at={(ticklabel cs:1.1)}},
tick label style={font=\footnotesize}]
\addplot [thick,dashed,name path=p3] coordinates {(-25,5) (25,5)};
\path [name path=left, %thick,draw=blue,-latex,
intersection segments={of=p1 and p3,sequence=L2}];
\path [name path=right, %thick,draw=red,-latex,
intersection segments={of=p2 and p3,sequence=L1}];
\addplot [pattern=north east lines,pattern color=blue,opacity=.8] fill between [
of=left and right,reverse=false];
\addplot+ [only marks,mark=*] coordinates { (-25,5) (25,5)}
node [pos=0,above right] {\footnotesize\textcolor{blue}{$B=(-25,5)$}}
node [pos=1,above left] {\footnotesize\textcolor{blue}{$A=(25,5)$}};
\node [right] at (11,3) {$r=x=y^2$};
\end{axis}
\end{tikzpicture}
\end{document}


to get the first output above.

• Amazing explanation. Incredible how some miracles could appear in LaTeX life!! – Sigur Dec 1 '18 at 17:51
• @Sigur Your comment seems to imply that there is a life without LaTeX. Really? ;-) – marmot Dec 1 '18 at 17:56
• Well, at least sometimes I move to a parallel life, Python life!!! lol – Sigur Dec 1 '18 at 17:58

in your particular cae you can define single path for curve sqrt(abs(x)) and with this somehow simplify diagram code:

\documentclass[margin=3.141592]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{intersections, patterns}

\begin{document}
\begin{tikzpicture}[lbl/.style={font=\footnotesize,text=blue}]
\begin{axis}[grid,
%axis equal=false,
xmin=-30,   xmax=30,
ymin=-.5,   ymax=6,
minor tick num=1,
axis lines = middle,
tick label style={inner sep=2pt, font=\footnotesize},
%
every axis plot post/.append style={thick},
samples=100
]
name path=p1] {sqrt(abs(x))};
every mark/.append style={solid,scale=1.2},
mark=*]   coordinates   {(-25,5) (25,5)}
node [pos=0,lbl, above right] {$B=(-25,5)$}
node [pos=1,lbl, above  left] {$A=( 25,5)$};
pattern color=blue,opacity=.8]
fill between [of=p1 and p3];
%
\node [right] at (11,3) {$r=x=y^2$};

• If you really want to simplify things here, just kick out all the fillbetween and name path stuff, and just do \addplot [draw=none,domain=-30:30,pattern=north east lines,pattern color=blue,domain=-25:25] {sqrt(abs(x))};. However, I thought this was not the point of the question. – marmot Dec 1 '18 at 20:05
• @marmot, as i understood the question, the point is remove doted line between point B and coordinate origin. and yes, your suggestion (with deleted domain=-30:30) gives even shorter code with desired result. however your code is real "miracle" (+1) and probably useful in more generalized cases when you can not use simpler solution mentioned in your comment above. – Zarko Dec 1 '18 at 20:19