def ellipse(numPoints, genX=np.linspace, HALF_WIDTH=10, HALF_HEIGHT=6.5):
    xs = 10.*genX(-1,1,numPoints)
    ys = 6.5*np.sqrt(1-(xs**2))
    return(xs, ys, "-")

sqrt(0) = 0
6.5*sqrt(1- (-1**2)) = 0

>>> import numpy as np
>>> np.sqrt(-1)
nan