Add PlotKey::Type2D for bar plots

[DJDuque]

Add an XBar and YBar to generate bar plots. These should have an internal structure for the bar width and bar shift keys.

It should be basically the same as the time I implemented Type2D::Smooth(f64):
659f09d

Should the internal structure just be (f64, f64)? Or maybe something like:

pub struct bar {
    bar_width: Option<f64>,
    bar_shift: Option<f64>,
}