For this code, the filling between curves overlaps half of the arrows line width as shown along the vertical arrow. So, how to make sure that it doesn't overlap any boundary path?
Additionally, for \tikzfillbetween, I would like to know how make it accept more than two surrounding paths. For example, how to tell \tikzfillbetween that the area to fill is formed by the paths Paxis, Raxis and curve?
\documentclass[border=2mm]{standalone}
\usepackage{pgfplots,tikz}
\usetikzlibrary{shapes.geometric,hobby,arrows.meta}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}[x=1.5mm,y=1.5mm]
\draw[ultra thick, ->, name path=Paxis] (0,0) ++(0,15) -- ++(0,-10) coordinate (tipParrow);
\draw[ultra thick, ->, name path=Raxis] (0,0) ++(0,15) -- ++(10,0) coordinate (tipRarrow);
\draw [ultra thick,name path=curve] [use Hobby shortcut] (tipParrow) ++(0,2) .. ++(2,1) .. ++(2,4.5) .. ([shift={(-2,0)}]tipRarrow);
\tikzfillbetween[ of=curve and Raxis, split] {red};
\end{tikzpicture}
\end{document}



\tikzfillbetweenthat I need, for example, to fill the area between three paths ofPaxis,Raxisandcurve? – Diaa Jun 28 '18 at 08:56\pgfplotsset{set layers}somewhere. If that does not work, please see section 4.27.2 of the pgfplots manual. Or are you using plain TikZ. (In that case I never got that message, so I'd need an MWE to say more.) – Aug 26 '18 at 22:36\tikzfillbetween[of=curve and PRaxes, on layer=background, split] {red}. However, commenting\scoped[on background layer]doesn't change the output for me, which makes me wonder what its meaning is. – Diaa Aug 26 '18 at 22:37\scoped[on background layer]to put fill in background, otherwise it partly cover axis lines. – Zarko Aug 26 '18 at 22:54