How can i add a filter on my graph where the Y axis is at some kbps on a specific period of time?

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I have applied throttling using wondershaper on some packets and i would like to have as a filter on every I/O graph, the downlink rate on Y axis in order to make my monitoring easier. (e.g. I want the downlink rate set the first 50sec at 0,3Mbps and then increase at 10Mbps for 250sec). How to draw a line in my already existed graph, in order to see the rate changes.