It looks like you are trying to extend the fitted curve to an “x” value 1,000,000 and find a corresponding “y” value for a specific “x” value (R_f = 151.3).
We can use the “fitresult” to predict the “y” values of the extended range of “x” values. The “feval’ function in MATLAB can be used for this and can also be used to get the “y” value for a specific “x” value (in this case, R_f = 151.3).
Here is something we can do to achieve the same:
function [fitresult, gof] = FXVol(x, y)
% ... [your code] ...
% Extend the range of x values up to 1000000
extended_x = linspace(min(xData), 1000000, 1000);
% Evaluate the fitted model at the extended range of x values
extended_y = feval(fitresult, extended_x);
% Plot the extended fit with data
figure('Name', 'Fredlund-XIng(1994)-Vvspsi');
plot(extended_x, extended_y, 'r-', 'LineWidth', 2); % Extended fit
hold on;
plot(fitresult, xData, yData); % Original data and fit
set(gca, 'XScale', 'log');
xlim([min(xData), 1000000]);
% ... [plotting code] ...
% Find the y value corresponding to R_f = 151.3
R_f = 151.3;
Y_point = feval(fitresult, R_f);
disp(['The y value corresponding to R_f = ', num2str(R_f), ' is: ', num2str(Y_point)]);
end
You can refer to the following MathWorks documentations to get more details on the “fevel” function:
