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Contourf is plotting weird lines. How to remove them?

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MDias
MDias on 22 May 2020
Commented: MDias on 23 May 2020
Hi,
I'm using hist3 to get bin centers and the elements in each bin. After this I use contourf to plot the data. Unfortunatelly the outputed contour figure has small vertical and horizointal black lines that don't seem to make sense. Please see lines inside red circles for examples:
My code is as follows:
edges = {0:0.2:13 0:0.2:13}; % edges for hist3
insetPos = [.52 .53 .35 .35];
[n,c] = hist3(X,'edges',edges);
figure
heatmap(n) % FEX
% contourmap
axes('Position',insetPos)
box on
contourf(c{1},c{2},n)
I belive the black lines are part of the contour lines, yet they appear where there are no boundries. If I remove the contour lines they also go away.
contourf(c{1},c{2},n,'LineColor','none') % no contour lines
Any tip on how I could keep the contour lines without drawing the "intruding" lines?

Accepted Answer

Kelly Kearney
Kelly Kearney on 22 May 2020
When you let contour choose contour levels automatically, it doesn't always choose the levels you really want. In this case, it chooses integer values, which may not be the best way to contour all-integer data. You might have better luck getting the look you want if you specify between-integer levels, e.g.
contourf(c{1}, c{2}, n, -0.5:10.5)
  1 Comment
MDias
MDias on 23 May 2020
Thanks Kelly!
Like Walter pointed out I didn't carefully at n...or at contourf for that matter. This does what I need.

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More Answers (2)

KSSV
KSSV on 22 May 2020
Use pcolor with shading interp instead of contourf.
  3 Comments
MDias
MDias on 22 May 2020
The unwanted bits of the contour (small black lines highlighted above) are still there.

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Walter Roberson
Walter Roberson on 22 May 2020
Look at n(24,9) and see that it is 1 there, corresponding to x=1.7, y=4.7; n(23,9) is also 1. That is an island of two 1's surrounded by 0's. With the contour levels happening to correspond to the integers, there should be an island there.
spy(n) and you will see there are a number of small islands.
  2 Comments
MDias
MDias on 23 May 2020
Thanks Walter!
Now this makes sense, I did not look carefully at n...

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