# using Matlab to plot density contour for scatter plot

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Keqiao Li on 30 Jun 2015
Edited: Cris LaPierre on 29 Oct 2020
Hi guys, I'm trying to use Matlab to plot the density contour for the following scatter plot. The bins will be 1X1 box. Within my limited understanding the density contour should be indicated how many points fall into the bins(correct me if I'm wrong), but I still cannot find a way to do the density plot. Need some help to solve this problem. Thanks! Mike Garrity on 30 Jun 2015
Scatter doesn't do any binning. It's showing each individual data element.
Perhaps this approach does what you want?
x = randn(1,1000);
y = randn(1,1000);
n = hist3([x', y']);
pcolor(n) Andrea Ackroyd on 3 Dec 2018
I am commenting on this old post because I am doing a very similar thing, but achieving some unexpected results.
I am making a histogram2 (tile view) of one dataset, which is time samples of two variables. On the same axes I plot a contour of a map which comes from different data (essentially a target map for the time based data which is in the histogram).
The unusual behaviour is that the colour scale of the hist2 appears to be affected by having the contour on the same axes.
If it is noteworthy, I am plotting the contour first, with a solid colour for all contour lines (not using colormap). Then I plot the histogram second.

### More Answers (3)

Walter Roberson on 30 Jun 2015
Assuming that your data starts at 0 and contains fractional values and is to be binned by 1 x 1, and that your coordinates are in X and Y,
xidx = 1 + floor(X(:));
maxxoff = max(xidx) - 1;
yidx = 1 + floor(Y(:));
maxyoff = max(yidx) - 1;
counts = accumarray([yidx, xidx], 1); %remember Y corresponds to rows
imagesc(counts, 'XData', 1/2+[0 maxxoff], 'YData', 1/2+[0 maxyoff]);
hold on
contour(1/2+(0:maxxoff), 1/2+(0:maxyoff), counts)
The 1/2 offsets have to do with referencing the centers of the boxes.

Elena De Angelis on 14 Jan 2016
Edited: Walter Roberson on 14 Jan 2016
Hi, I'm trying to do the same but my result is not what I expect, could someone tell me please what i'm doing wrong?
x = dNBR_filt_CHANGE;
y = NBR_post_CHANGE;
scatter(x,y,'r.')
hold on
[n,c] = hist3([x, y]);
contour(c{1},c{2},n) Hamdullah OZOGUL on 1 Feb 2018
Edited: Hamdullah OZOGUL on 1 Feb 2018
You just have to transpose the n to have the correct order.
contour(c{1},c{2},n')

Jos? Manuel Amigo on 11 Nov 2017
PCx = rand(2000,1); PCy = rand(2000,1);
bins = 50; Msize = 10;
[N C] = hist3([PCx,PCy],[bins bins]);
CX = C{1}; CY = C{2};
N2 = N; N2(N2 == 0) = []; Nunique = unique(N2);
colors = jet(length(Nunique));
for i = 1:length(PCx) if isnan(PCx(i)) PCxnew(i,1) = NaN; PCynew(i,1) = NaN; J(i,1) = NaN; else whichoneX = find(min(abs(CX - PCx(i))) == abs(CX - PCx(i))); PCxnew(i,1) = CX(whichoneX(1)); whichoneY = find(min(abs(CY - PCy(i))) == abs(CY - PCy(i))); PCynew(i,1) = CY(whichoneY(1)); J(i,1) = sub2ind([bins,bins],whichoneX(1),whichoneY(1)); end end
for i = 1:bins for j = 1:bins temp = sub2ind([bins,bins],i,j); Jthese = find(J == temp); if ~isempty(Jthese) Ntemp = N(temp); Nthis = find(Nunique == Ntemp); plot(PCx(Jthese),PCy(Jthese),'.','color',colors(Nthis,:),'Markersize',Msize); hold on; end end end hold off;
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Jos? Manuel Amigo on 11 Nov 2017
Sorry for the bad pasting. But if you copy-paste, it works