Is there a way to mark tangent lines on a graph plotted using the curve fitting app?

Hi,

I've used the curve fitting app to plot a 2D graph using a set of data points. Is there a way to mark tangent lines on specific points on this curve fit?

Thanks

Answer 1

[Shivang]

Hi Delika,

I understand that you've using the Curve Fitter app to fit a curve to a set of data. And you would like to mark the tangent lines at particular points on this curve.

This can be done manually by using the equation of the fit created by the Curve Fitter app. The steps to be follwed are listed below:

• Export the fit from the Curve Fitter app to the Workspace, using the "Export" option in the app's toolstrip. Assume the fit object is named "fitResult".
• Once we have the equation of the fit, we need to find the derivative. I will assume that the fit is polynomial, for which the derivative is already well known. You can use symbolic computation to find the equation of the derivative in general. Refer to the documentation of the "diff" function in MATLAB: https://www.mathworks.com/help/releases/R2020a/symbolic/diff.html

        % Assuming a quadratic fit: ax^2 + bx + c
        coeffs = coeffvalues(fitResult);
        a = coeffs(1);
        b = coeffs(2);
        % For a quadratic fit, the derivative is: 2ax + b                                                                                                                                                                                                                                                                                                                                                                                                                                             

• Now we can calculate the slope of these tangent lines, and plot these lines at the specified points.

        x_points = [5, 10]; % Points at which you want the tangent lines
        x_range = linspace(min(x_data), max(x_data), 1000); % Assuming x_data is your data
        
        figure;
        hold on;
        plot(x_data, y_data, 'ro'); % Plot original data points
        plot(x_range, fitResult(x_range), 'b-'); % Plot the fit
        
        for i = 1:length(x_points)
            x_tan = x_points(i);
            y_tan = fitResult(x_tan); % Evaluate the fit at the tangent point
            
            % Calculate the slope of the tangent line
            slope = 2*a*x_tan + b; % Adjust this based on the actual derivative
            
            % Define the tangent line equation: y = mx + c
            % Calculate c using the point (x_tan, y_tan)
            c = y_tan - slope*x_tan;
            
            % Plot the tangent line
            x_tangent = linspace(x_tan-1, x_tan+1, 100); % Adjust the range as needed
            y_tangent = slope*x_tangent + c;
            plot(x_tangent, y_tangent, 'k--', 'LineWidth', 2); % Plot each tangent line
        end
        
        hold off;

Hope this helps!

-Shivang