There are two axes. One is responsible for field.
And the second ?

first the field is rescaled into 0-1 range using (Fit Range (Unclamped) and Ramp Interval parameters) if they are used

then X axis represents such rescaled value of the source field
Y axis represents new corresponding value which will become the result

so if X is 0.5 while Y is 1 it means that where rescaled source was 0.5 output value will be 1, it essentially allows you to shape the value using lookup curve over the value of the original field

Thanks