How do scientists use hotspot volcanism to tell the direction and speed
of a plate?

Since many hotspots are stationary in the mantle, geologists can use some hotspot chains to tell the direction and the speed a plate is moving. Although most hot spots occur far from plate boundaries, they offer a way to measure plate movement. This is because a hot spot generally stays in one place while the tectonic plate above it. At a hot spot, the heat from the plume partly melts some of the rock in the tectonic plate above it.

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero. 

An em wave does not require a medium, while a mechanical wave does.