I want to label a family of curves representing a simple family of mathematical functions : x -> x²+c where c will take various values between, say, -6 and 6.
I want to superpose the label directly on the curve (thus interrupting the trace, or at least opacifying it) and let a little white space at both sides of the mathematical formula which labels each curve to help the reading.
I would also want to reduce the size of fonts inside mathematical formulas. Here where I am, with the unvaluable help of Schrödinger's cat, thanks to him (cf. this question How to typeset text along the curve of a mathematical function with TikZ, I followed his suggestion to ask a new question to improve clarity) :
\documentclass[11pt,a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{arrows, arrows.meta, decorations.text, decorations.pathmorphing} % decoration.markings
\begin{document}
\begin{tikzpicture}[scale=.8,>=Stealth]
\def \xmin {-4}
\def \xmax {4}
\def \ymin {-6}
\def \ymax {6}
\draw[thin, opacity=0.2] (\xmin, \ymin) grid (\xmax, \ymax);
\draw[very thick, ->] (\xmin,0) -- (\xmax,0) node[below left] {$x$} ;
\draw[very thick, ->] (0,\ymin) -- (0,\ymax) node[below left] {$y$} ;
\clip (\xmin, \ymin) rectangle (\xmax, \ymax) ;
\draw[very thick, dashed, domain=-3:3,samples=200] plot (\x, {(\x)^2} ) ;
\foreach \c [evaluate=\c as \pc using {100-(\c+6)*9}, evaluate=\c as \ic using {abs(\xmax-\c)+.5}] in {-6,-4,-2,2,4} {
\draw[very thick, domain=\xmin:\xmax,samples=200, blue!\pc!red,
% text effects={path from text, characters={text along path, fill=gray!50}},
postaction={decorate,
decoration={text along path,
raise=-.5ex,
% fill=green % Let (Lua)LaTeX hang
text={|\scriptsize| $x {\mapsto} {x^2}+(\c)$},
text color=blue!\pc!red,
text align={align=right, right indent=\ic cm}
},
text effects={path from text, characters={text along path, fill=gray!50}},
} ]
plot (\x, {(\x)^2+\c} ) ;
} ;
\draw (0,0) node[shift={(-.3cm,-.3cm)}, fill=white, circle, inner sep=-.5pt] {$O$} ;
\coordinate (I) at (1,0); \draw (I) node {\small $|$} node[yshift=-.3cm, fill=white, inner sep=.5pt] {$1$} ;
\coordinate (J) at (0,1); \draw (J) node {$-$} node[xshift=-.2cm, fill=white, inner sep=.5pt] {$1$} ;
\end{tikzpicture}
\end{document}
I tried to use the TikZ library decocations.text with the "text effects along a path" decoration, but this seems to be unusable with maths...
Many thanks in advance for any ideas or improvements !
Update
Thanks one more time to Schrödinger's cat. I tweaked his idea and went to tcolorbox rather than contour. Here is the code :
\documentclass[11pt,a4paper]{article}
\usepackage{tcolorbox, tikz}
\usetikzlibrary{arrows, arrows.meta, decorations.text, decorations.pathmorphing} % decoration.markings
\begin{document}
\begin{tikzpicture}[scale=.8,>=Stealth]
\def\xmin{-4}
\def\xmax{4}
\def\ymin{-6}
\def\ymax{6}
\def\cmin{-6}
\def\cmax{5}
\draw[thin, opacity=0.2] (\xmin, \ymin) grid (\xmax, \ymax);
\draw[very thick, ->] (\xmin,0) -- (\xmax,0) node[below left] {$x$} ;
\draw[very thick, ->] (0,\ymin) -- (0,\ymax) node[below left] {$y$} ;
\clip (\xmin, \ymin) rectangle (\xmax, \ymax) ;
\draw[very thick, dashed, domain=-3:3,samples=200] plot (\x, {(\x)^2} ) ;
\def\mycommand#1{\tcbox[size=tight, on line, colback=white, colframe=white, boxrule=0em, opacityframe=1, opacityback=0]{\tiny $#1$}}
\foreach \c [evaluate=\c as \pc using {100-(\c+6)*9},
evaluate=\c as \ic using { (\cmax-\c)^1.55 } ]
in {\cmin,...,-1,1,2,...,\cmax} {
\pgfmathsetmacro{\localxmax}{sqrt(\ymax-\c)}
\ifnum\c<0
\def\mysign{-}
\else
\def\mysign{+}
\fi
\pgfmathtruncatemacro{\absc}{abs(\c)}
\draw[very thick, domain=-\localxmax:\localxmax,samples=200, blue!\pc!red,
postaction={decorate,
decoration={text effects along path,
text={x{\mapsto}{x^2}\mysign\absc{\hspace{\ic em}}},
raise=-.35ex,
text align={align=right},
text effects/.cd,
every character/.append style={character command=\mycommand}, text along path}
} ]
plot (\x, {(\x)^2+\c} ) ;
} ;
\draw (0,0) node[shift={(-.3cm,-.3cm)}, fill=white, circle, inner sep=-.5pt] {$O$} ;
\coordinate (I) at (1,0); \draw (I) node {\small $|$} node[yshift=-.3cm, fill=white, inner sep=.5pt] {$1$} ;
\coordinate (J) at (0,1); \draw (J) node {$-$} node[xshift=-.2cm, fill=white, inner sep=.5pt] {$1$} ;
\end{tikzpicture}
\end{document}
Which gives the following result :
One minor issue still persists : I cannot change the color of the formulas...
