function plot_curve(c, M, str)

% plot_curve - plot a 2d curve.
% 
% plot_curve(c, str, M);
%
%   Note that 'c' can also be a cell array of curves.
%   - 'c' is of size 2xm
%   - 'M' is a nxn image put in background (ranging in [0,1]