# Chaining gnuplot function calls in tikz, reverse plotting

I have a parabola drawn through A, B and C. There are three different equations: for the top darker border, the fill (this is a hack just using a wide line) and the lower border. It's not quite what I want.

I'd like to emphasize that there are two separate sections of the arch which are pinned together at B so I am trying to draw AB on its own. In red, I plot the function for the upper border from A to almost B, then use an arc to touch at B and reach to bottom border of the arch.

I would like to chain a function call to my statement (the last statement below) that will plot the formula of the lower border of the arch from B back to A.

I'm not sure of the syntax:

1. How do a I chain a function call, with a different domain from the first call in the statement?
2. When plotting the parabola of the bottom border of the arch from B back to A, I need to draw the parabola from right to left (B_x to A_x). Is this possible?
 % arch fill (central formula)
\draw[domain=4.2:13.825, color=LightCyan3, line width=0.375cm, line cap=rect] plot[id=1] function{-3*x*x/32+3*x/2};

% upper arch border
\draw[domain=3.85:14.187, color=LightCyan4, thick] plot[id=2] function{-.090215*x*x+1.4425*x+0.48342};

% lower arch border
\draw[domain=4.15:13.813, color=LightCyan4, thick] plot[id=3] function{-0.097567*x*x+1.5621*x-0.50220};

% incomplete red border
\draw[domain=3.85:7.75, draw=red, fill=none, very thick]plot[id=4] function{-0.090211*x*x+1.4425*x+0.48342}arc(90:-90:0.25);



Most likely I do not fully understand or appreciate what you want. Yes, you can reverse the plot, and I do not think you need gnuplot for such a simple function.

\documentclass[tikz,border=5pt]{standalone}
\usetikzlibrary{positioning,shapes.multipart,shapes,fit,decorations.pathreplacing}
\begin{document}
\begin{tikzpicture}
\draw[domain=4.2:13.825, color=cyan, line width=0.375cm, line cap=rect] plot[id=1] function{-3*x*x/32+3*x/2};

% upper arc border
\draw[domain=3.85:14.187, color=cyan, thick] plot[variable=\x]
({\x},{-.090215*\x*\x+1.4425*\x+0.48342});

% lower arc border
\draw[domain=4.15:13.813, color=cyan, thick] plot[variable=\x]
({\x},{-0.097567*\x*\x+1.5621*\x-0.50220});

% incomplete red border
\draw[draw=red, fill=none, very thick,fill=red!20]
plot[domain=3.85:7.75,variable=\x] ({\x},{-0.090211*\x*\x+1.4425*\x+0.48342})
arc(90:-90:0.25)
plot[domain=7.75:3.85,variable=\x] ({\x},{-0.097567*\x*\x+1.5621*\x-0.50220})
-- (3.85,{-0.090211*3.85*3.85+1.4425*3.85+0.48342});
\end{tikzpicture}
\end{document}


For fun, I also shaded the contour. If you don't like this, just remove the fill=red!20 statement and -- (3.85,{-0.090211*3.85*3.85+1.4425*3.85+0.48342}) (but not the semicolon).

• You understood perfectly. :) – dmorg Apr 21 '18 at 3:54