function phaseportrait(data_filename, XLABEL, YLABEL, TITLE)
%plots a phase portrait, i.e. the velocity of a particle as a function of its position


%first, load your data from the trajectory file: 
load(data_filename)

%The 2 loaded veriables are called X and X_DOT. 
%  -- X is a matrix where each column is the position evolution for a given
%	set of initial conditions. 
%  -- X_DOT is a matrix where each column is the velocity evolution for a given
%	set of initial conditions.

%plot also accept 2 matrices as arguments X_DATA, and Y_DATA or
%same size, and %plots as many curves as there are columns, the
%data in each column being the x-coordinates and the y-coordinates
%of the points to be plotted: 
plot(X, X_DOT)


%standard labeling and title: 
xlabel(XLABEL);
ylabel(YLABEL);
title(TITLE);