Select certain percentage of a plot

Hi all,

I was wondering, how can I select certain points which contains a % desired amount.

To make it a bit clearer, imagine a x,y plot where

x = [ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20]

y = [0 1 3 1 2 0 0 0 2 1 2 2 4 5 6 7 8 9 11 20]

I want to find that point where y reaches for example 10% of the sum of y (which should be about 8.4 in this case). This should be between x = 8 and x = 9.

How can I give Matlab a command to find it for a certain percentage and the value of the x-axis? And is it also possible to indicate the value in the plot itself?

Thanks in forward

Answer 1

[Kevin Phung]

x = [ 0 1 2 3 4 5 6 7 8  10 11 12 13 14 15 16 17 18 19 20];
y = [0 1 3 1 2 0 0 0 2 1 2 2 4 5 6 7 8 9 11 20];
percent = .1;
thresh = percent*sum(y);
[closest_dist, idx] = min(abs(x-thresh));
x_vals = x(ismember(x,x(idx)));
y_vals = y(ismember(x,x(idx)));

will give you your x and y values. This code takes into account duplicates.

as for indicating it in the plot, you can plot each xy pair as its own line object in a for loop

line_objs= gobjects(1,numel(x));
for i = 1:numel(x)
    p = plot(x(i),y(i),'*b')
    line_objs(i) = p % store in graphics array
    hold on;
end

%change color of the specific point(s):
line_objs(idx).Color = [1 0 0] % to red

*note: your x and y are different lengths. you should double check it and make sure they match

[Toan Vu]

Yes I got it, thanks! However, for the second part, if I run that, then it will only give a single point in the plot, without the actual plot and that point is not at the right spot, how can I fix that?

[Kevin Phung]

oops, i forgot to add a 'hold on' into the loop, I've edited my ans

Answer 2

[aara]

You can use a for loop add all the totals together incrementally and check when its above your desired value:

x = 1:20;
y = [0 1 3 1 2 0 0 0 2 1 2 2 4 5 6 7 8 9 11 20];
total = 0;
percent = 10; %set percentage.

for i=1:length(y)
    total = sum(y(1:i)); %this adds more y values together with each iteration
    if total > sum(y)*percent/100 
        disp(x(i)) %x is only displayed when the sum values of y are bigger than percentage of y 
        break
    end
end

Or if you need to integrate for area under the curve then:

You can use the trapz function in matlab (this gives different results) :

x = 1:20;
y = [0 1 3 1 2 0 0 0 2 1 2 2 4 5 6 7 8 9 11 20];
total = 0; %this is to add the total area under the curve
percent = 10;

for i=1:length(y)
    total = total+trapz(y(1:i)); %with each iteration, the integration area increases
    if total >= sum(y)*percent/100 %if the area under the curve passes your required percent then it would display corresponding x value and stop the loop.
        disp("X-value: "+ x(i))
        break
    end
end