Confusion about valve openings?

I have a confusion about this valve opening graph that why exhaust valve opening is shown at 180 degrees at crank (Bang part of the stroke) first, when SUCK (Intake) is the first step of the engine cycle in a four stroke engine? I'm a noob so please bear with me.

It really doesn't matter where we start drawing the 4 cycles of the process. We've drawn it starting with the power stroke as it makes it easier to visualise the valve opening and closing events which then occur in the centre of this drawing. It's a continuous cycle though so just keeps repeating.

Thanks Andre for your response.