Let R be a vector of the row coordinates of the selected pixels, and C be a vector of the corresponding column coordinates. Let your image be I. Then
reds = I(sub2idx(size(I), R(:), C(:), 1 * ones(length(R),1));
greens = I(sub2idx(size(I), R(:), C(:), 2 * ones(length(R),1));
blues = I(sub2idx(size(I), R(:), C(:), 3 * ones(length(R),1));
Now you could plot in whatever way seems appropriate. For example,
X = 1 : length(reds);
plot(X, reds, 'r', X, greens, 'g', X, blues, 'b');
