Something like this should work (using random data and using the second line from each plot call in the legend - adjust as necessary):
% random data
X = rand(1,10);
Y = rand(1,10);
x_mtf = rand(1,10);
y_mtf = rand(1,10);
% Get the line handles returned from plot()
h_mtf = plot(X,Y,'k*', x_mtf, y_mtf, 'k-')
hold on
% random data
X = rand(1,10);
Y = rand(1,10);
x_source = rand(1,10);
y_source = rand(1,10);
% Get the line handles returned from plot()
h_source = plot(X,Y,'b*', x_source, y_source, 'b-')
hold on
% give legend() only the lines you want to use in the legend,
% along with their names:
legend([h_mtf(2) h_source(2)],{'MTE1' 'source report'})
