For a harmonic oscillator, the graph between momentum p and displacement q would come out as an ellipse. Assuming the mass isconstant, the momentum displacement graph is of the sameshape as the velocity-displacement graph. Just imagine a pendulumswinging backward and forward for one cycle. Consider its x coordinate as its velocity at different points through the cycle. When the pendulum is at the maximum (max.) displacement on the right, x ispositive max. and v = 0, this is the rightmost point on the ellipse. When the pendulum is at its midpoint moving to the left, x = 0 and vis max. and negative, this is the bottom point of the ellipse. When the pendulum is max. displacement on the left, x is negative max. and v= 0, this is the leftmost point on the ellipse. When the pendulum is at its mid-point and moving to the right, x = 0 and v is max and positive, this is the top point of the ellipse.