What you describe sounds as if either X_1 or Y_1 are scalars. In such a case, requesting . or o will draw markers, but there would be no finite (x,y) pairs to draw lines between.
Lower probability is if X_1 or Y_1 happens to be such that it has infinite or nan entries between every finite entry.
x = 1:5;
y = [1 nan inf 4 nan];
subplot(2,1,1)
plot(x,y)
xlim([0 5]); ylim([0 5])
subplot(2,1,2)
plot(x,y,'o-')
xlim([0 5]); ylim([0 5])
The first plot does not show any lines because lines are only drawn if there are adjacent finite coordinates.
