# Pgfplots colorbar relative to x values

Usually, when a colorbar is attached to a graph, the colorbar shows color changes in relation to the values in the y-axis. Is there a way to display color changes according to the x-axis values? For example, in the colorbar you can see how the color changes according to the milliliters?

\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{textcomp}
\usepackage[italian]{babel}
\usepackage[
a4paper,
margin=15mm,
bindingoffset=2mm,
heightrounded,
]{geometry}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{float}
\usepackage{pgfplots}
\usepackage{xcolor}
\usepgfplotslibrary{external}
\tikzexternalize
\pgfplotsset{width=7cm,compat=1.14}

\begin{document}
\begin{flushleft}
\begin{tikzpicture}
\begin{axis}[
colorbar,
width=12cm,
xmin=0, xmax=21,
ymin=0, ymax=14,
xlabel=ml,
ylabel=pH,
xmajorgrids=true,
ymajorgrids=true,
grid style=dashed,
ytick={0,1,2,3,4,5,6,7,8,9,10,11,12,13,14},
xtick={0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20},
]

\pgfplotsset{
colormap={redblue}{color(0pt)=(white); color(62pt)=(white); color(63pt)=(magenta); color(100pt)=(magenta)}
}
coordinates {
(1,3.61)(2,3.99)(3,4.16)(4,4.33)
(5,4.52)(6,4.64)(7,4.80)(8,4.91)(9,5.06)(10,5.24)(10.5,5.38)(11,5.49)(11.5,5.67)(12,5.82)(12.2,5.99)(12.4,6.09)(12.6,6.46)(12.8,6.98)(13,7.35)(13.2,10.92)(13.4,11.22)(13.6,11.37)(13.8,11.50)(14,11.58)
(14.5,11.78)(15,11.83)(15.5,11.91)(16,11.97)(17,12.08)(18,12.13)(19,12.18)(20,12.22)
};
\end{axis}
\end{tikzpicture}
\end{flushleft}
\end{document}


This can be achieved by using the tikz option point meta=x, which you can put with the other options after \begin{axis}.
If you also want your colorbar to be horizontally below the x-axis, you can use the option colorbar horizontal instead of colorbar.
• For the record: point meta is a PGFPlots key (and not a TikZ key). – Stefan Pinnow May 15 '17 at 18:08