2

I have been trying to make a 3D plot of a certain bivariate normal distribution but that is far from ideal at the moment. The code I have been using is 

\begin{tikzpicture}
\centering
\begin{axis}[
width=6in,
height=4in,
title=Bivariate Normal Distribution,
axis lines=left,
grid=both,
]
\addplot3[samples=50,surf,faceted color=blue]
{1/(2 *pi* sqrt(1-0.9^2))* exp(-(x^2+y^2-2*0.9*x*y)/(2*(1-0.9^2))};
\end{axis}
\end{tikzpicture}

which produces this

enter image description here

This is is not what I would like though and me messing with the different graphical parameters of \addplot3 does not lead to a more pleasant result for the eyes. Ideally, the surface I would like to produce is something like this

enter image description here

I really don't mind the shape, I can change it without any problems, it's the graphics I am having trouble with. Could you please tell me which parameters I need to change to get something like the picture above?

Thank you.

JohnK
  • 354

1 Answers1

6

The main difference between the image of the 3rd party tool and pgfplots is that you have a considerably more involved example for pgfplots: the sampling density is way too low to draw a rotated asymmetric distribution in cartesian coordinates.

Options include:

  1. do not use a rotated distribution if it does not matter anyway. Solutions how to do it are shown in all detail in Draw a bivariate normal distribution in TikZ, in this case it is a duplicate.
  2. maybe polar coordinates are better suited (I did not try it)
  3. increase the sampling density

Here is what comes out of approach (3):

\documentclass{standalone}

\usepackage{pgfplots}

\pgfplotsset{compat=1.12}


\begin{document}

\begin{tikzpicture}
\begin{axis}[
width=6in,
height=4in,
title=Bivariate Normal Distribution,
axis lines=left,
grid=both,
]
\addplot3[samples=150,surf,shader=interp]
{1/(2 *pi* sqrt(1-0.9^2))* exp(-(x^2+y^2-2*0.9*x*y)/(2*(1-0.9^2)))};
\end{axis}
\end{tikzpicture}
\end{document}

enter image description here

I made some obvious and some non-obvious changes to your code and I would like to discuss them here:

  1. \centering inside of a tikzpicture has no effect.
  2. I added compat=1.12 and compiled the picture with lualatex. This is much faster than any older compat level or pdflatex.
  3. I fixed a syntax error in your math expression: the last ')' is missing (causing the lua backend to bail out and fall back to the slow TeX implementation which lives with the syntax error).
  4. I used shader=interp since samples=150 results in too many grid lines when used with faceted.
Community
  • 1
Christian Feuersänger
  • 55,840
  • Hello, thank you for your answer. It's the surface of the density that I would like to imitate somehow, that is why the other answer does not cover it. I do not mind the color, there is the other answer for that. – JohnK Apr 04 '15 at 10:23
  • OK. Do you need more input in order to fulfil your task? Otherwise you can accept the answer. – Christian Feuersänger Apr 04 '15 at 14:07
  • I would like to create the kind of surface of the picture I have posted, aside from the color. Is that possible? – JohnK Apr 04 '15 at 14:15
  • Since my image already is kind of that surface, I would like to know what you have in mind. Grid lines? Tick labels? black mesh on top of some colored surface? A precise parameterization of a gaussian with the parameters of the image? – Christian Feuersänger Apr 08 '15 at 17:04
  • Hello Christian, it's ok I have figured it out. Thank you for your help. – JohnK Apr 08 '15 at 17:06