function [n,vx,p] = Vregression(axis,varargin)

%Varargin is in the form data1,pwr1,a1,b1,data2,pwr2,a1,b1,data3,pwr3,a1,b1,....
%This function will create a regression line for the linear region in the
%vx vs sample number plot in the analysis of a Stuck Bead.

%data1,.... is the file gained from the instrument

%a1,b1,..... represent the initial and final value for the linear region
%for each dataset

%pwr1,.... is the power at which the data is gathered


 if strcmp(axis,'y') %uses the y component data
        for i=1:4:length(varargin);
            %plot vy vs sample number
            power=cell2mat(varargin(i+1));
            data=cell2mat(varargin(i));
            n=data(:,1);
            vy=data(:,3);
            figure(i);
            plot(n,vy);
            xlabel 'Time (ms)';
            ylabel 'Vy';
            title (['Vy vs. Time at ' num2str(power) 'mW']);
            grid;
            hold on;

            %Regression line
            a=cell2mat(varargin(i+2));
            b=cell2mat(varargin(i+3));
            n_1=data(a:b,1);
            vy_1=data(a:b,3);
            p=polyfit(n_1,vy_1,1);
            f=polyval(p,n_1);
            p_1=p(1);
            p_2=p(2);
            plot(n_1,f,'r');
            legend('data',['equation fit (time = ' num2str(a) ' to ' num2str(b) '): ' num2str(p_1) 'x + ' num2str(p_2)]);
            hold off;
            end;

    elseif strcmp(axis,'x') %uses x component data
            for i=1:4:length(varargin);
            %plot vx vs sample number
            power=cell2mat(varargin(i+1));
            data=cell2mat(varargin(i));
            n=data(:,1);
            vx=data(:,2);
            figure(i);
            plot(n,vx);
            xlabel 'Time (ms)';
            ylabel 'Vx';
            title (['Vx vs. Time at ' num2str(power) 'mW']);
            grid;
            hold on;

            %Regression line
            a=cell2mat(varargin(i+2));
            b=cell2mat(varargin(i+3)); 
            n_1=data(a:b,1);
            vx_1=data(a:b,2);
            p=polyfit(n_1,vx_1,1);
            f=polyval(p,n_1);
            p_1=p(1);
            p_2=p(2);
            plot(n_1,f,'r');
            legend('data',['equation fit (time = ' num2str(a) ' to ' num2str(b) '): ' num2str(p_1) 'x + ' num2str(p_2)]);
            hold off;
            end;
    else
        disp('wrong input format')
 end
end