Help with Plotting the Envelope of a Scatter Plot in MATLAB

17 次查看（过去 30 天）
Zac Lee
Zac Lee 2025-8-4
评论： Zac Lee 2025-8-5
Hello everyone,
I'm trying to plot the envelope of a scatter plot in MATLAB. However, when I use the envelope function, it returns the same plot without highlighting the upper or lower bounds.
This plot was created using a scatter plot, and when I attempt to filter it to retain only the extremum values, I end up with just a single slice of the data.
Has anyone encountered a similar issue or knows how I could extract and plot the envelope correctly (in red), as shown in the reference image?
Thanks in advance!
  2 个评论
Zac Lee
Zac Lee 2025-8-4
移动：Star Strider 2025-8-4
Dear @Star Strider, here my x,y datas. The code is simply the classical roots of a 2nd degree polinomial equation.
Catalytic
Catalytic 2025-8-4
编辑：Catalytic 2025-8-4
The sampling density is very non-uniform, and makes it hard to see why the red boundary is appropriate in certain places. Why couldn't someone assume the following boundary drawing be valid? You need to achieve denser sampling throughout the shape for any automatic boundary detector to have any hope.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Matt J
Matt J 2025-8-4
Ran in:
load xy
x=x(:); y=y(:);
[G,yg]=findgroups(y);
xmin=splitapply(@min,x,G);
xmax=splitapply(@max,x,G);
[yg,is]=sort(yg);
xmin=xmin(is);
xmax=xmax(flip(is));
X=[xmin;xmax]; Y=[yg;flip(yg)];
scatter(x,y,'.');hold on
plot(X,Y,'r-x'); hold off

更多回答（2 个）

Star Strider
Star Strider 2025-8-4
编辑：Star Strider 2025-8-4
The boundary function or related functions can probably help with this.
Since the data appear to have stepwise y-values, another option would be to get the x-values at each y-value. This could be a problem between y-values 0.02 to 0.027 since those x-values are discontinuous. Depending on how your data are presented, that could be either straightforward or somewhat difficult.
Without your data and the code you used to create that plot, I cannot experiment with either of these approaches.
EDIT -- Corrected typographical errors.
.
Catalytic
Catalytic 2025-8-4
编辑：Catalytic 2025-8-4
Ran in:
load xy
x=x(:)/1000; y=y(:);
r=median(diff(unique(y)));
shp=alphaShape([x,y],r*.8);
[~,P]=boundaryFacets(shp);
scatter(x,y,'.b'); hold on
plot(P(:,1),P(:,2),'r.--'); hold off
  4 个评论
显示 2更早的评论
Catalytic
Catalytic 2025-8-4
Ran in:
If you increase the sampling density in the region y<0.005, as I mentioned in my comment above, then there would be not need to incorporate a second method.
load xy
x=x(:); y=y(:);
r=median(diff(unique(y)));
s=1000;
shp=alphaShape([x/s,y],r*.8);
[~,P]=boundaryFacets(shp);
P(:,1)=P(:,1)*s;
scatter(x,y,'.b'); hold on
plot(P(:,1),P(:,2),'r.--'); hold off
●

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Scatter Plots 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by