Finding points along a plot in Matlab
I have the following plot and a file of the data which creates that plot. I would like to have Matlab find the following points for me:
- [y,x] for peak noted by the 100% line
- [x] for where the plot crosses the y=0 line
- [x] for where y is 50% and 20% of the peak found in part 1.
Are there any add-on tools or packages which people are aware of which can help me accomplish this? I need to do this for a collection of plots so something reasonably automated would be ideal.
I can certainly do the programming and calculation parts in Matlab, it's just a matter of being able to load in the data file, matching it to a curve or function, and find the various [x,y] co-ordinates.
UpdateHere's a sample dataset I am testing with:
x-values y-values
-9.90056E+01 4.08949E-03
-8.58224E+01 -2.52083E-02
-7.26381E+01 3.39976E-02
-5.94526E+01 -6.90023E-02
-4.62659E+01 8.37428E-02
-3.30781E+01 -1.74139E-01
-1.98891E+01 2.09784E-01
-6.69894E+00 7.43248E-01
6.49238E+00 6.08539E-02
1.96849E+01 -1.64983E-01
3.28785E+01 8.34376E-02
4.60733E+01 -6.71712E-02
5.92692E+01 7.21458E-02
7.24664E+01 -2.20038E-02
8.56646E+01 3.44554E-02
9.88641E+01 -9.46063E-042 Answers
$\begingroup$Maybe this code can help you:
function mse
x = linspace(0,10,100);
y = sin(x)+x-(x/5).^2;
hold off
plot(x,y)
yMax = max(y);
xMax = x(find(y==yMax));
hold on
plot(xMax,yMax,'*g')
y90 = yMax*0.9;
plot(x,y90,'-k')
for i=2:length(x) if y(i-1)<=y90 && y(i)>=y90 || y(i-1)>=y90 && y(i)<=y90 plot(x(i),y(i),'og') plot(x(i-1),y(i-1),'og') end
end
y50 = yMax*0.5;
plot(x,y50,'-k')
for i=2:length(x) if y(i-1)<=y50 && y(i)>=y50 || y(i-1)>=y50 && y(i)<=y50 plot(x(i),y(i),'og') plot(x(i-1),y(i-1),'og') end
end
endHere $x,y$ are my data. Finding the maximum is easy, but for the 90% or 50% values I iterated through the data. As we do not have continous data we check, whether the 90% or 50% values lies between to points. The result is shown below. If you only want to highlight a single point instead of the two you could perform some (linear) interpolation.
EDIT: This would highlight all points that match the X% criteria. If you only want to highlight the closest points to the maximum, you could change the loop to something like
for i=find(y==ymax):length(x)Which finds the point on the right and for the left side
for i=find(y==ymax):-1:length(x)To mark only one point you could use a break command.
EDIT2: The following code performs the same thing as the for loops, but it is shorter.
y90 = yMax*0.9;
xId= find(diff(sign(y-y90))~=0);
plot(x(xId),y(xId),'*k')
plot(x(xId+1),y(xId+1),'*k')EDIT3: For your date you need to interpolate between the two values. Replace the loop with something like
for i=2:length(x) if y(i-1)<=y90 && y(i)>=y90 || y(i-1)>=y90 && y(i)<=y90 plot(x(i),y(i),'og') plot(x(i-1),y(i-1),'og') xl = x(i-1); yl = y(i-1); xr = x(i); yr = y(i); xMid = (y90-yl)*(xr-xl)/(yr-yl)+xl; plot(xMid,y90,'or'); end
endYou can find the maximum point using the $\tt max$ and $\tt find$ commands in Matlab:
$\tt ymax = max(y);$
$\tt xmax = x(find(y == ymax));$
The other points do not exist in your data, i.e. there is probably no point that hits exactly the 20% line or the 50% line. For these you would need to interpolate.
$\endgroup$ 4
