You have made a mistake. You wrote:
"normally:
m = xy
m/x = y"
That's wrong. You mean:
m = y/x
y = mx
_______________________________
The question is best stated as "How would a graph of acceleration vs 1/mass for a constant force look?"
Rahul's answer is short and clear. But to help, let me give an example with numbers.
__________________________________
The easiest way to see what happens is to make up some simple values. For example:
Suppose F = 10N. This remains fixed.
F = ma. So a = F/m. We can write this as: a = F * (1/m)
If m=10kg
1/m = 1/10 = 0.1 kg^-1
a = F * (1/m)= 10 * 0.1 = 1m/^2
If m=5kg
1/m = 1/5 = 0.2 kg^-1
a = F * (1/m) = 10 *0.2 = 2m/^2
If m=1kg
1/m = 1/1 =1.0 kg^-1
a = F * (1/m) = 10 * 1.0 = 10m/^2
So using '1/m' values on the x axis and 'a' values on the y axis, the points will be:
(0.1, 1)
(0.2, 2)
(1.0, 10)
which is a straight line passing through the origin. The gradient is 10 (which is the force).
______________________
Here is a more detailed explanation:
F = ma, so a = F/m. Write this as:
a = F * (1/m) (equation 1)
You should see that because F is a constant, equation 1 is really the same as Y=MX (I've used capitals as we don't want to confuse M (gradient) with m (mass).)
Compare a = F * (1/m) to Y=M * X
a corresponds to the variable Y.
F corresponds to the constant M (which is the gradient)
(1/m) corresponds to the variable X.
So a graph of a vs. (1/m) will have gradient equal to F.