You're right, its not relevant to a single timestamp, this will calculate it for a series of values. The y axis is the values from each timer sample. The x axis just ups by 1 so it plots them at even spacing. The slope of that graph is the rate of change for the timer values.  For normal timers that are sending similar values all teh time the slope should be around 0.0 all the time indicating little or no change in the values.  For ever increasing values the slope is the rate that the values are increasing.