You can simply use the negative x axis to show the negative class size.
However, in the example you have mentioned, the class size is not negative. The class size is 0.5 i.e. 0.5 - 0 or 1 - 0.5. And for this, you can simply use the mid points of the 2 classes on x axis i..e 0.25 and 0.75 and show their respective frequencies on the y axis.