Extrapolating from linear fit

I have a code, and it works, except is there a way to extend the linear fits past the data they are fitted to? Currently, it plots a line over my data. How can I specify the limits of the linear fit (I'd like to extrapolate). Here is my code:

close all
clear
clc
[D,S] = xlsread('C:\PATH\kvsl.xlsx','500K');

one_over_l = D(:,10);
one_over_k = D(:,11);

ballistic_x = one_over_l((1:7),1);
ballistic_y = one_over_k((1:7),1);

diffusive_x = one_over_l((8:17),1);
diffusive_y = one_over_k((8:17),1);

figure(1)
plot(ballistic_x, ballistic_y,'o')
hold on

   P = polyfit(ballistic_x,ballistic_y,1);
      yfit1 = P(1)*ballistic_x+P(2);
      hold on;
      plot(ballistic_x,yfit1,'r-.');

   Q = polyfit(diffusive_x,diffusive_y,1);
      yfit2 = Q(1)*diffusive_x+Q(2);
      hold on;
      plot(diffusive_x,yfit2,'b-');

plot(diffusive_x, diffusive_y,'x')
grid on
ylabel('1/\kappa (W/m.K)^{-1}')
xlabel('1/L (\mum^{-1})')

Answer 1

[madhan ravi]

https://www.mathworks.com/help/matlab/ref/interp1.html#btw2a2x

https://www.mathworks.com/help/matlab/ref/interp1.html#btwp6lt-1-extrapolation

Answer 2

[Bruno Luong]

" Currently, it plots a line over my data. How can I specify the limits of the linear fit (I'd like to extrapolate)."

You get it all wrong, polyfit returns coefficients, for later usage it doesn't care the interval used for fit. You can plot in larger interval if you want

Instead of

yfit1 = P(1)*ballistic_x+P(2);

which is similar to stock function

yfit1 = polyval(P,ballistic_x);

Simply extend x

minx = min(ballistic_x);
maxx = max(ballistic_x);
dx = 0.1*(maxx-minx);
xextended = [minx-dx, maxx+dx];
yextended = polyval(P,xextended );

plot(xextended, yextended ,'r-.');