How to extract single x-value given y threshold

 I want to extract x when y is set to a threshold (i.e., if y >= 1.5e12...). Reference the attached image. Essential the x1, and y1 will be fed into my function, it will be fitted, and then the output should be a single value (x given y, or the dashed/solid line in the image). New to matlab, so anything helps!

%%Curve fit plotting %%

x1 = timeStamps(1:60); % taking timestamps from 1 - 120 given smoothed y1 values 
y1 = smooth(tic_lin(1:60),'sgolay',1);

% Find coefficients for polynomial (order = 4 and 6, respectively)
fitResults1 = polyfit(x1',y1, 7);

% evaluate the fitted y-values
yplot1 = polyval(fitResults1,x1');

% interpolates to find yi, the values of the underlying function Y at the points in the vector or array xi. x must be a vector. 
Time_points = interp1(yplot1, x1', yplot1);

figure( 'Name', 'Curvefit1_poly' );
h = plot(x1', y1);%smoothed-points
hold on;
plot(x1', yplot1);%polyfit points
hold on;
plot(Time_points, yplot1, '*r');%interpolated points of x given y

%given y-threshold, output x(corresponding time_point).
broken = false;

while broken == false
    if yplot1 >= 2024671226502.99
        index = find(yplot1);
        xDesired = x1(index);
        broken = true;
    else
        disp("next iteration through");
    end
end

Answer 1

[jonas]

I always recommend InterX for accurate and precise intersections

Although, in many cases you can just find the first y-value above the threshold.

id=find(y>=thres,1, 'first')
out=x(id)

[Dane Dewees]

Thank you!