Warning due to use of deprecated default_features

[andrewdavidmackenzie]

In my build I noticed this, so I thought I'd mention it:

cargo build --features "gui"
warning: `default_features` is deprecated in favor of `default-features` and will not work in the 2024 edition
(in the `plotters` dependency)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.36s