# Fill area under curve using colourmap and Tikz

I'd like to use a colourmap like hot2 in this plot, where red occurs at x=0, and green occurs at x=1, but I can't find any way of doing it.

Here's what I have so far:

    \begin{tikzpicture}
\begin{axis}[
axis lines=left,
grid=major,
no marks,
xmin=0, xmax=1, xlabel=$E$,
ymin=0, ymax=1, ylabel=$\lambda$, ylabel style={rotate=-90}
]
\addplot+[smooth,blue,name path=A] {1 - (1 - x)^2}; % actual curve
\addplot+[draw=none,name path=B] {0};     % “fictional” curve
\addplot+[green] fill between[of=A and B,soft clip={domain=0:1}]; % filling
\end{axis}
\end{tikzpicture}


Welcome to TeX-SE! We usually begin our examples with \documentclass, end them with \end{document} and do not put non-standard characters in the code that prevents others from copying it. The following is an example of an MWE.

\documentclass[tikz,border=3.14mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\newsavebox\backbox
% where '\result' is inserted as last argument:
\expandafter\tempb\expandafter{\result}%
\begin{document}
\begin{tikzpicture}
\begin{axis}[set layers,view={90}{0},
axis lines=left,
grid=major,
no marks,
xmin=0, xmax=1, xlabel=$E$,
ymin=0, ymax=1, ylabel=$\lambda$, ylabel style={rotate=-90}
]
\addplot+[smooth,blue] {1 - (1 - x)^2}; % actual curve
\begin{pgfonlayer}{pre main}
\clip plot[variable=\x,samples=25,domain=0:1] (\x,{1 - (1 - \x)^2})
|-cycle;
\node at (0.5,0.5){\usebox\backbox};
\end{pgfonlayer}
% filling
\end{axis}
\end{tikzpicture}
\end{document}


• Perfect thank you! – Jordan Mackie Apr 4 '19 at 18:02
• It works , but I do get an error: ! Undefined control sequence. <argument> \backbox l.41 \sbox\backbox{\pgfuseshading{tempshading}} % ? – Jordan Mackie Apr 4 '19 at 18:12
• @JordanMackie Sorry, these statements seem to contradict each other: "it works" and "I get an error". I get this very output from running pdflatex on the above code, of course without error message. – user121799 Apr 4 '19 at 18:44
• It does produce the same output, but compilation always trips up on that error, so it's not a major – Jordan Mackie Apr 4 '19 at 22:01
• @JordanMackie An error is major, but I do not get one if I compile the above on an updated TeXLive 2018 distribution with pdflatex. Which TeX installation and compiler do you use? – user121799 Apr 4 '19 at 22:12