how to use matplotlib contour()?

[Grant Edwards]

On 2006-05-24, Robert Kern <robert.kern at gmail.com> wrote:

> Yes, unfortunately, much of the documentation was written by people who were
> very familiar with the Matlab interfaces that these functions are emulating.

Since I've never used matlab, I'm a bit clueless.

>> For example one parameter is specied as "an array".  No clue as
>> to how many dimensions or what the axis are.
>> 
>> In another place it says "the X,Y parameters specify the (x,y)
>> coordinates of a surface".  _How_ do they specify the surface?
>> Are they just equal length lists of x and y coordinates that
>> specify len(X) points.  Or do they specify a len(X) x len(Y)
>> grid of points?
>
>> Why would my Z values be a 2D array?
>
> contour() only does contouring on gridded data.

That's what I was beginning to suspect.  What confused me was
that if it required gridded data, I expected the input parameters to
specify a grid (e.g. for a 5x7 grid, the X parameter would be a
vector of 5 values, and the Y parameter would be a vector of 7
values) rather than just lie on a grid.

> If you want to handle scattered datapoints, you will have to
> do some interpolation.
>
>   http://www.scipy.org/Cookbook/Matplotlib/Gridding_irregularly_spaced_data

Thanks, that looks like exactly what I need.  My next step was
actually going to be to use the Delaunay triangulation module
to interpolate the data onto a much finer grid.

> So X, Y, and Z are all 2-D arrays laid out corresponding to
> the grid that you have sampled. I thought the contour_demo.py
> example was reasonably clear on this, but if you didn't get it
> to run, then I can see why you wouldn't have read it.

I did delete some code that was attempting to label the graph,
and then it ran.  The examples do use gridded data, but when I
changed them to non-gridded data, it seemed to run fine.

> Talking about this on matplotlib-users will probably get these
> problems fixed faster:
>
>   https://lists.sourceforge.net/lists/listinfo/matplotlib-users

After I got the demos to run, it became apparent that the
contour functions don't do what I want anyway, so it's all moot
at this point.

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