Simple depth-cueing using graduated colours in PCA

Sequence points in the PCA display are coloured as per sequence ids. Currently points 'behind' the centre are dimmed (Color.darker()). Instead could graduate the colour from darker() to brighter() based on where the z-value lies between z-min and z-max. This gives a more convincing 3-D illusion (at the expense of some perceptual accuracy when multiple values of the same colour are assigned to different sequences).