MP := proc(P0,r,y,m)
   P0*r/m*(1+r/m)^(y*m)/((1+r/m)^(y*m)-1)
end;

plot_mortgage:= proc(P0, r, y, m)
   local principal, M, n, i, j, k;
   n := y * m;
   principal := table();
   principal[0] := P0;
   M := MP(P0, r, y, m);
   for i from 1 to n do
       principal[i]:=(1+r/m)*principal[i-1] - M
   od;
   plot({[ [j, principal[j]] $j=0..n ],
         [ [k, k*M] $k=0..n ]},
        style=LINE) 
end;