To correctly graph this, you need to set up a simple equation and table of values. Luckily, this equation is dead-simple; I'll define y as the total cost and x as the number of water bottles sold.

$y=1.5x$

Since 1.50$ is the cost for one bottle, multiplying that with your variable that defined the amount of bottles, x, gets you the total, y. Now that we have a basic equation, we can begin plugging in values.

Recall that a function is basically just something that takes in a value and returns another one; in our case, it takes the amount of bottles and returns the total cost. Now, plug in the x-values present on the graph (specifically only whole numbers, since you can't have a half bottle). I can't make a proper table but I'll make do.

x y

--------

0 0

1 1.5

2 3

3 4.5

4 6

5 7.5

-----------

Great, now that you have a table of values all you have to do is plug them into the graph, which I've attached. It's pretty crude since I drew it in mspaint but I'm sure you get the point at this point.