Alerting : Want to get alerted when their is a drop in between a mention range


Am having difficulty to setup alerting for my graph,

Please find below example

So with regard to the above example ive pasted, i would like to get an alert when we have a small or big drop as you can see in the picture.

How will it be possible for me to setup the same alert.

Right now i can do a min and a max which will alert for example if the graph goes below 20 or above 190, but for those in between drops i dont know how i can setup same.

Thanks to help