Hi In a three-dimensional graph with x, y, z are. For example, a point with an entry in the chart is displayed. And can change with a change in the graph is obtained. Desired program, please write to me. Thank you.

a=input('a='); x=[0,34.5]; y=[0,19.5]; z=[19.5,34.5]; u = gradient(x); v = gradient(y); w = gradient(z); scale = 0.1; quiver3(x,y,z,u,v,w,scale); view([56 54]);