1. An apparatus, comprising: a simulator that includes one or more computing circuits and that is configured to simulate a system and to propagate at least one state of the simulated system through time in response to a value of a parameter; a plotter configured to generate a representation of a plot having coordinate dimensions respectively corresponding to at least the parameter and to a characteristic of another state of the simulated system, a representation of a plot point located on the plot at a coordinate corresponding to the value of the parameter and to a value of the characteristic of the other state, and a representation of at least one level set located on the plot at a coordinate corresponding to a level-set value of the characteristic of the other state; and a determiner configured to determine a next value of the parameter in response to the location of the representation of the plot point relative to the representation of the at least one level set.