Why does my graph plot show a horizontal line?

My Matlab ver is 2017b

I want to get rid of that horizontal line or at least know what it meant.

This is my code.

    figure
    plot(num(:,8),num(:,10)) 
    title('xyz');
    xlabel('x');
    ylabel('y');
    grid on;
   

[Alex Mcaulley]

I guess that your last 'x' value is 0 (and the previous one is ~21), then you obtain the horizontal line

[XH K]

oh yes.. thanks. I see it now. I didnt realised the line is connected to the last value.