[BUG] Uneven mesh spacing

[EFanZh]

Describe the bug

I noticed uneven mesh spacing when I uses date values as X values:

Notice the spaces between vertical lines are not always the same.

To Reproduce

use chrono::{TimeZone, Utc};
use plotters::chart::ChartBuilder;
use plotters::drawing::IntoDrawingArea;
use plotters::prelude::SVGBackend;
use std::error::Error;
use std::fs::File;
use std::io::Write;

fn main() -> Result<(), Box<dyn Error>> {
 let mut svg = String::new();

 ChartBuilder::on(&SVGBackend::with_string(&mut svg, (1000, 600)).into_drawing_area())
 .build_cartesian_2d(Utc.ymd(2018, 2, 1)..Utc.ymd(2021, 8, 1), 0..100)?
 .configure_mesh()
 .x_labels(10)
 .y_labels(10)
 .draw()?;

 File::create("output.svg")
 .unwrap()
 .write_all(svg.as_bytes())
 .map_err(Into::into)
}

Version Information

I am using plotters 0.3.1.