Compiled example
----------------
![Example](3d-gradient-colored.png)


Creation
--------
* Make sure that `gnuplot` is installed and in your PATH.
* You can let Wolfram|Alpha calculate you gradients (http://www.wolframalpha.com/input/?i=gradient+of+x%2F%28e%5E%28x%5E2%2By%5E2%29%29)
* See http://en.wikibooks.org/wiki/LaTeX/Colors for colors


Credits
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Jake helped me to remove the small white border at the left side which was
caused by the arrows.
See [Why does PGFplots plot more than the desired range in a 3D plot?](http://tex.stackexchange.com/a/227173/5645).