[Matplotlib-users] Splitting arrays into chunks that satisfy a condition?

[Drain, T. R (392P) <the...@jp...>]

I have three arrays (x,y,z).  I want plot x vs y and draw the line segments differently depending on whether or not z is positive or negative.  So I'm trying to split the x,y arrays into chunks depending on the value of z.  Using numpy.where, I can find the indeces in z that satisfy a condition but I can't figure out an efficient way (other than brute force) to split the array up into continuous chunks.  Does anyone know of a numpy trick that would help with this?

Here's a simple example:

# index:    0  1  2  3   4  5  6  7 8 9
z=numpy.array([-1,-1,-1, 1, -1,-1,-1, 1,1,1] )
x=numpy.array([-2,-3,-4, 2, -5,-6,-7, 3,4,5] )

# Want: xneg = [ x[0:3], x[4:7] ], xpos = [ x[3:4], x[7:10] ]
xneg = [ [-2,-3,-4], [-5,-6,-7] ]
xpos = [ [ 2 ], [ 3, 4, 5 ] ]

idxneg = numpy.where( z < 0 )[0]
# == [ 0,1,2, 4,5,6 ]
idxpos = numpy.where( z >= 0 )[0]
# == [ 3, 7,8,9 ]

Thanks,
Ted