PGFplots, two colours under and above curve

Question

I'm using PGF plots and would like to fill two colours to my plot. One colour indicates a positive value, while the other colour for negative values. What I have so far is one colour for the entire plot.

\begin{tikzpicture}[scale=1.4]
\begin{axis}[no markers, scale only axis,ylabel={SOI}, axis on top=true, axis x line=below,axis y line=middle]
\addplot {sin(x)} \closedcycle;
%\addplot [red, fill] table[x=Year,y=INDEX,col sep=comma] {data/SOIvRainfall.csv} \closedcycle;
\end{axis}
\end{tikzpicture}

Answer 1

You could use the min(a,b) and max(a,b) functions with the plot as one and zero as the other parameter to get the positive and negative parts of a function. For plots from tables / files you'll have to use y expr and \thisrow{}:

Code

\documentclass[parskip]{scrartcl}
\usepackage[margin=15mm]{geometry}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
\begin{axis}
[   xlabel=degree,
    ylabel=sin,
    no markers,
]
\addplot[fill=cyan,draw=blue!50!black,thick] table[x=deg,y expr={max(\thisrow{val},0)}] {
deg val
-400    -0.64
-380    -0.34
-360    0.00
-340    0.34
-320    0.64
-300    0.87
-280    0.98
-260    0.98
-240    0.87
-220    0.64
-200    0.34
-180    0.00
-160    -0.34
-140    -0.64
-120    -0.87
-100    -0.98
-80 -0.98
-60 -0.87
-40 -0.64
-20 -0.34
0   0.00
20  0.34
40  0.64
60  0.87
80  0.98
100 0.98
120 0.87
140 0.64
160 0.34
180 0.00
200 -0.34
220 -0.64
240 -0.87
260 -0.98
280 -0.98
300 -0.87
320 -0.64
340 -0.34
360 0.00
380 0.34
400 0.64
} \closedcycle;

\addplot[fill=orange,draw=orange!50!black,thick] table[x=deg,y expr={min(\thisrow{val},0)}] {
deg val
-400    -0.64
-380    -0.34
-360    0.00
-340    0.34
-320    0.64
-300    0.87
-280    0.98
-260    0.98
-240    0.87
-220    0.64
-200    0.34
-180    0.00
-160    -0.34
-140    -0.64
-120    -0.87
-100    -0.98
-80 -0.98
-60 -0.87
-40 -0.64
-20 -0.34
0   0.00
20  0.34
40  0.64
60  0.87
80  0.98
100 0.98
120 0.87
140 0.64
160 0.34
180 0.00
200 -0.34
220 -0.64
240 -0.87
260 -0.98
280 -0.98
300 -0.87
320 -0.64
340 -0.34
360 0.00
380 0.34
400 0.64
} \closedcycle;
\end{axis}
\end{tikzpicture}

\end{document}

Result