Doubt in plot, help me

Hello guys, how are u? I'm a doubt in plot graphics in matlab. How i put a line in axis x in matlab, 'cause when i put values in variables x and y, dont have a line in axis x. For example: Look the picture:

I want so:

Help me pelase

Answer 1

[Image Analyst]

Try this

grid on;
line(xlim, [0,0], 'Color', 'k', 'LineWidth', 3);