[BUG] Time for axes does not work on single day with different hours

[lobneroO]

Describe the bug
In the book there is an example using Utc times with just dates. This works fine.
If however you use data that is all on the same day with different times of day, all times are rendered as "00:00Z".

To Reproduce
MWE:

use plotters::prelude::*;
use chrono::{Utc, TimeZone};

fn main() {
  let root_area = BitMapBackend::new("out.png", (600, 400))
    .into_drawing_area();
  root_area.fill(&WHITE).unwrap();

  let start_date = Utc.with_ymd_and_hms(2019, 10, 1, 8, 0, 0).unwrap();
  let end_date = Utc.with_ymd_and_hms(2019, 10, 1, 20, 0, 0).unwrap();

    let mut ctx = ChartBuilder::on(&root_area)
        .set_label_area_size(LabelAreaPosition::Left, 40)
        .set_label_area_size(LabelAreaPosition::Bottom, 40)
        .caption("MSFT daily close price", ("sans-serif", 40))
        .build_cartesian_2d(130.0..145.0, start_date..end_date )
        .unwrap();

    ctx.configure_mesh().draw().unwrap();

    ctx.draw_series(
        LineSeries::new(
            (0..).zip(DATA.iter()).map(|(idx, price)| {
                let day = (idx / 5) * 7 + idx % 5 + 1;
                let date = Utc.with_ymd_and_hms(2019,10, 1, day, 0, 0).unwrap();
                (*price, date)
            }),
            &BLUE,
        )
    ).unwrap();

}
const DATA: [f64; 14] = [ 137.24, 136.37, 138.43, 137.41, 139.69, 140.41, 141.58, 139.55, 139.68, 139.10, 138.24, 135.67, 137.12, 138.12];

This is the example given in the book, but modified:

• axes are swapped
• instead of different days, everything is on the same day with different hours

Version Information
cargo.toml:
plotters = "0.3.6"