Read about xticks for labelling x1, x2 along the axes. I have taken random points for plotting. You can take your required points to plot striaght line and the curve.
% Striaght line
L = rand(2,2) ;
% Curve passing through above two points
p = polyfit(L(:,1),L(:,2),3) ;
xi = linspace(min(L(:,1)),max(L(:,1))) ;
yi = polyval(p,xi) ;
%plot
figure
hold on
plot(L(:,1),L(:,2),'r')
plot(xi,yi,'b')
xlabel('x')
ylabel('y')
% Add text
text(mean(L(:,1)),mean(L(:,2))+0.02,'p(x)','FontSize',20)
text(mean(xi),mean(yi)-0.02,'f(x)','FontSize',20)
