Without looking at the graph you are plotting it is difficult to gauge the context of your question. However your question points to the plotting of a linear equation on a graph. As a value for x and a value for y are not specifically given we can assume x is equal to zero when y is equal to zero. As zero is a constant, half of zero would still be zero. To put it more simply, if you have zero apples, you cannot then take half of the apples you have away, as you have no apples to begin with. Your task will then be to apportion values to y in order to gain the points for your graph, and to be able to give value to, and plot, x. For example if you assume that the value of x increases as the value of y increases then you could draw yourself an easy chart to help to plot your graph. All you would have to do is assume x and y are the same number in the above question, and then subtract the value of x in half to reach your graph conclusion. For instance, if y is equal to two then x must be equal to one (as you would have to half the number of Y to get x as suggested in the question). It would therefore stand to reason that when y is equal to 10, x is equal to five; if y is equal to 15, x is equal to 7.5, if y is equal to 18.5 then x would be 9.25 and so on. Without looking at the graph you are plotting I am afraid that it is impossible to be more accurate with the advice, and advice here is based on assumption.
An easier solution (in my opinion) would be to start at (0,0). From there, go right 2 spaces and up 1 space. Draw a line through those two points and you are done. This method is the Slope-Intercept form, meaning you are using the Slope (1/2 (Rise/Run)) and the Y-Intercept (which in this case is the origin (0,0)). I will not get into the other methods.
Pick some numbers for x (like 0, 2, 4). Now for each of those figure out what y would be.
For example, if x=0, y=1/2*0 or y=0. Now you have a point to plot (0,0). Do that for a bunch of x's so you have a bunch of points and connect the dots!
This is a linear equation. For any linear equation, you will get a straight line. For each "x" value (the domain) must have a "y" value(the range).
You can use any numbers to be the value of 'x". For example, if x= 2, y will equal to 1. You can write these values as an ordered pair, (2,1). If x=0, y must be 0. Write as an ordered pair, (0,0). As long as you have two points, you can connect these points to make a straight line. When you draw the line, make sure that your line does not stop at the two points but extend the line by drawing two small arrows to each end.
They are two curves in first quadrant and third quadrant respectively.