Host colors are not always unique

[bestchai]

Currently, ShiViz is generating colors for hosts using d3.scale.category20(), which gives us only 20 unique colors. If the number of hosts is greater than that, the user will see two hosts using the same color.

[Issue created by mhnnunes: 2016-06-22]
[Last updated on bitbucket: 2017-04-22]

[Comment created by grahamstl: 2017-04-22]
The new colours should be darker, rather than lighter; and perhaps we should remove some of the less readable colours in the process of fixing it.