function computeNormalBetweenXYpoints, xPoints, yPoints

nPoints = n_elements(xPoints)
if nPoints ne n_elements(yPoints) then message,'Number of x and y points do not match'

normal = fltarr(2,nPoints-1)

v2 = [0.0, 0.0, 1.0]
for i=0L,nPoints-2 do begin
  v1 = [xPoints[i+1] - xPoints[i],yPoints[i+1] - yPoints[i],0]
  v1 = v1/norm(v1)
  v3 = crossp(v1,v2)
  v3 = v3/norm(v3)
  normal[0,i] = xPoints[i] + v3[0]
  normal[1,i] = yPoints[i] + v3[1]
endfor

return, normal
end

pro testComputeNormalBetweenXYpoints

theta = findgen(36)*10
x = 2*cos(theta*!dtor)
y = sin(theta*!dtor)

plot,x,y,ystyle=1,xstyle=1,psym=-1,xrange=[-2,2],yrange=[-2,2]
normals = computeNormalBetweenXYpoints(x,y)
for i=0,n_elements(x)-2 do begin
  plots,[x[i],normals[0,i]],[y[i],normals[1,i]]
  ;plots,[normals[0,i]],[normals[1,i]],psym=1
endfor
wshow
return & end