Wired behavior of contour plot

đź“Š Plotly Python
1

It looks like when the points are not given in a “square manner”, the 2d contour plot looks strange.
When 6 points (0,0) (1,0) (2,0) (0,1) (1,1) (2,1) are given, the plot looks perfect.

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1.PNG1851Ă—846 85.5 KB

However, when the 6 points are (0,0) (1,0.001) (2,0) (0,1.001) (1,1) (2,2), the plot look much too different than the previous one.
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2.PNG1854Ă—863 95.5 KB

Is it using linear interpolation to generate the contour plot?
Where does Plotly handle the plot contour algorithm?
I would appreciate any help you offer.
Thank you.

Here is the code I used:
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@xutianqish To plot a contour with Plotly you have 2 options:

  1. either you give only the z values as a list of lists or a numpy array of shape (m,n);
  2. or you provide x, y as lists, numpy arrays, etc, and z as in the case 1

For details and examples see: https://plot.ly/python/reference/#contour and https://plot.ly/python/contour-plots/

In your example x, y and z are intended to be simple lists, but they aren’t because nodes is a list of lists, NOT a numpy array, and you cannot slice it in this way because it leads to the following error:

----> 1 nodes[:,0]

TypeError: list indices must be integers, not tuple
1 Like
3

Hi empet. Thank you for your detailed reply and references. However, the problem still exists after I converted the data type.
Basically, I’m trying to do the visualization of FEM analysis but the plot looks strange when the mesh is not grid-based. (e.g. triangular mesh or some random quadrangle mesh) In the txt file I have nodeID, X, Y, and value of the nodes.

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1.PNG1871Ă—898 77.9 KB

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2.PNG1877Ă—875 87.5 KB

I suspect that plotly doesn’t handle the data in the way I want, that’s why I would like to have a look into the algorithm.
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5.png1892Ă—1810 400 KB

The first image is what I get when the mesh type is not a perfect rectangle.
The second image is what I get when the mesh type is perfect rectangle, which matches the result of a commercial product.

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Plotly works with rectangular meshes. The contour points are computed via marching squares algorithm, and they are interpolated by the centripetal Catmull-Rom method.

1 Like
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Thanks a lot empet! I will look into plotly library or try some other plotting libraries instead.
Cheers,
Xutianqish