How is constant velocity indicated on a motion map

Constant velocity refers to constant rate of change in position with time

which means equal change in position of object in equal interval of time

so it we are plotting a motion diagram where we will represent a position - time graph then

the motion plot must be a straight line with a constant slope

here we know that

so velocity is slope of the position time graph which remains constant

Similarly if the graph is plot against velocity and time then we will again represent it with straight line but this time the straight line must be of zero slope because here velocity is not changing with time.