If it points upwards, it has a positive slope. (goes up from left to right)
If it points downwards, it has a negative slope. (goes down from left to right)
If it is perfectly horizontal, its slope is 0.
If it is perfectly vertical, its slope is undefined.
You can always count on your independant variable (x-axis) being horizontal and your dependant variable (y-axis) being vertical, so these do not change.
