close all;
clear all;
clc;

dataFolder = 'c:\Tan2.xlsx';

excelObj = actxserver('Excel.Application');
excelWorkbook = excelObj.workbooks.Open(dataFolder);
worksheets = excelObj.sheets;
numSheets = worksheets.Count;  


for i=1:numSheets
    
    data =readtable(dataFolder, 'Sheet', i);
    sheet = get(worksheets,'item',i);
    disp(['Sheet',num2str(i), ' : ', sheet.Name]);

    V = data.V(~isnan(data.V));   %t
    I = data.I(~isnan(data.I));   %y

    %I = T1.I ;
    %V = T1.V ;
    dI = gradient(I );
    dV = gradient(V);

    Vi = 1.3;                       % Choose Voltage Value To Calculate Slope
    Ii = interp1(V, I, Vi );
    dVi = interp1(V, dV, Vi )
    dIi = interp1(V, dI, Vi)

    Slope = dIi/dVi
    YIntercept = Ii - Slope * Vi
    XIntercept = -YIntercept / Slope
    figure
    plot(V,I )
    hold on
    plot(Vi, Ii, 'p ')
    plot(xlim, xlim*Slope+YIntercept, '-r ')
    plot(XIntercept, 0, 'sr ')
    hold off
    grid
    xlabel('V ')
    ylabel('I ')
    text(Vi, Ii, sprintf('$I = %.3f \\times V %+.3f \\rightarrow \\ $', Slope, YIntercept), 'Horiz','right', 'Vert','middle', 'Interpreter','latex ')
    text(XIntercept, 0, sprintf('$%.4f V \\downarrow $', XIntercept), 'Horiz','right', 'Vert','bottom', 'Interpreter','latex ')
    ylim([0 max(ylim)])

end