The equation y = ax describes the graph of a line. if the valise of a is positive , the line:

Given a linear equation of the form y=ax, 

like: y=5x, y=-14x, y=-7x, y=(1/2)x etc,

then this equation represents a line.

If :

i) a>0, the line is increasing
ii)a<0, the line is decreasing
iii) a= 0, then y=ax becomes y=0, which is the equation of the x-axis.

the line is increasing, that is it rises continuously as it moves to the right.

Could you make it more clear?