What represents instantaneous velocity on a graph?

-- If it's a graph of speed/time, then the speed at any instant of time is the 'y' value of the graph at that time.

-- If it's a graph of distance/time, and it's a straight line, then the speed at any time is the slope of that line.  If the graph has peaks and valleys and dips in it, then the speed at any time is the slope of a short piece of line that just touches the graph at that time.

(To be very technical, you can never read velocity from a graph, and you can't even draw a graph of velocity. Velocity is a speed and the direction of the speed, and it's almost impossible to combine all of that information in one graph.)