Improve color scheme

[dtolnay]

I did not find a library for good graph color schemes so there is a quick attempt here. There is a lot of room for improvement so we need to switch to an approach that is proven to work well for graphs.

[dtolnay]

I posted a request for implementation to make this easier to do: dtolnay/request-for-implementation#12.

[pyfisch]

There is now https://crates.io/crates/colourado

[dtolnay]

This got fixed by just using D3's color choices.

cargo-tally/src/index.html

Line 37 in c65c7e3

var color = d3.scaleOrdinal(d3.schemeCategory10);