How do I incorporate the hysteresis effect in a control flow?

I still don't really understand what you are trying to do.

Is the variable Y the upper limit for your temperature comparison and X is the lower limit??

So the sort of checks you want to do are... IF temperature > Y
and another check is IF temperature < X

This is a very similar question to what you posted in...

Are you trying to incorpoarte a hysteresis effect in your control system?