Draw a line through two points

Question

Using MatLab, I know how to create a line segment connecting two points using this code:

line([0 1],[0 1])

This draws a straight line segment from the point (0,0) to the point (1,1).

What I am trying to do is continue that line to the edge of the plot. Rather than just drawing a line between these two points I want to draw a line through those two points that spans the entire figure for any set of two points.

For this particular line and a x=-10:10, y=-10:10 plot I could write:

line([-10 10], [-10 10]);

But I would need to generalize this for any set of points.

Answer 1

What about

  function = long_line(X,Y,sym_len)
       dir = (Y-X)/norm(Y-X);
       Yp = X + dir*sym_len;
       Xp = X - dir*sym_len;
       line(Xp,Yp);
  end

being sym_len one half of the expected length of the plotted line around X?

Answer 2

1. Solve the line equation going through those two points:

   y = a*x + b;
   

   for a and b:

   a = (yp(2)-yp(1)) / (xp(2)-xp(1));
   b = yp(1)-a*xp(1);
   

2. Find the edges of the plotting window

   xlims = xlim(gca);
   ylims = ylim(gca);
   

   or take a edges far away, so you can still zoomout, later reset the x/y limits.
   or if there is no plot at the moment, define your desired edges:

   xlims = [-10 10];
   ylims = [-10 10];
   

3. Fill in those edges into the line equation and plot the corresponding points:

   y = xlims*a+b;
   line( xlims, y );
   

4. And reset the edges

   xlim(xlims);
   ylim(ylims);
   

There is one special case, the vertical line, which you'll have to take care of separately.