To create a stateflow logic which decreases the Lamp intensity gradually you can use the "after" function in stateflow to execute a state after some elapsed time.
I have designed this stateflow chart which switches state based on the switch "s" to three states:
The Lamp Dimming state gradually decreases the lamp intensity each second and the state changes when intensity crosses certain threshold.
This is the output of Lamp intensity signal from the stateflow chart.
To increase the smoothness of decay you have decrease the delay in the "after" function and also decrease the step size in the fixed step solver. I have also attached the Simulink model for better understanding.
You can learn more about "after" function by accessing the official MathWorks Documentation by executing the following command.
web(fullfile(docroot, 'stateflow/ref/after.html'))