PCA points not dimmed when rotated about y axis

PCA display dims (darkens) points that are behind the plane of the centre (have negative z coordinate). The test is in RotatableCanvas.drawScene:
    float z = sp.coord[1] - centre[2];
    if (z < 0) { /* darken */ }
looks wrong, should be sp.coord[2] - centre[2].
Sure enough, if you drag the mouse left and right to rotate around the y axis (y value remains constant), no dimming / undimming is seen, although points are conceptually getting rotated to the back or front.