How to find x values of specified y point on the graph ?

Let say i have two array x[1,2,3,4,5] and y[6, 7,8,9,10 ] i want to find the x values of the 6.34 on the graph. I want to put marker for that point is there any way to do it?

Answer 1

[Voss]

x = [1,2,3,4,5];
y = [6,7,8,9,10];

y_point = 6.34;
x_point = interp1(y,x,y_point);

plot(x,y);
hold on
plot(x_point,y_point,'r.')

[Zack Trahem]

Thank you for quick response. this look like what i looking for. However, my y vector is not sorted in assending order and unique. I attached figure.

[Sebastian]

Thank you so much Voss holy

[erin]

I seem to be having a problem where I input the code but the point isn't actually on the line of the graph

[Voss]

@erin: You may need to interpolate over multiple segments separately:

x = [1,2,3,4,5,6,7,8,9,10];
y = [6,7,8,9,10,9.5,8.5,7.5,6.5,5.5];

y_point = 6.34;

idx = find(diff(sign(y-y_point)));
n = numel(idx);
xi = zeros(1,n);
yi = zeros(1,n);
nx = numel(x);
for ii = 1:n
    tmp = max(1,idx(ii)-1):min(nx,idx(ii)+1);
    xi(ii) = interp1(y(tmp),x(tmp),y_point);
    yi(ii) = y_point;
end

plot(x,y,'o-');
hold on
plot(xi,yi,'r.')

Answer 2

[Sam Chak]

Hi Zack,

You can use interp1 technique.

Answer 3

[Star Strider]

Using the supplied .fig file —

F = openfig('graph1.fig');
Lines = findobj(gca, 'Type','line');
x = Lines.XData;
y = Lines.YData;

yval = 0.0100277;                                               % Choose A Value Within tThe Range Of 'y'
yxi = find(diff(sign(y-yval)));
for k = 1:numel(yxi)
    idxrng = max(1,yxi(k)-1) : min(numel(y),yxi(k)+1);
    xv(k) = interp1(y(idxrng), x(idxrng),yval);
    yv(k) = yval;
end

% figure
% plot(x, y)
hold on
plot(xv, yv, 'rs', 'MarkerSize',10)
hold off

EDIT — (26 Nov 2024 at 12:05)

Ran code here. This was not possible when this post first appeared.

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