Added axis labels formatting; Height and width limits changed; Axis are now rendered before the graph to prevent artifacts on colored diagrams;

[uandere]

This pull request allows a user to format the labels of the axis:

    Chart::new_with_y_range(200, 50, 0.0, (end - start).num_days() as f32, 0.0, 25_000.0)
        .linecolorplot(&Shape::Continuous(Box::new(|x| 1000.0 * (5.0 * (0.5 * x).sin() + 0.05 * x) + 9000.0)), rgb::RGB { r: 10, g: 100, b: 200 })
        .x_label_format(LabelFormat::Custom(Box::new(move |val| {format!("{}", start + Duration::days(val as i64))})))
        .y_label_format(LabelFormat::Value)
        .display();

This will result in following chart:

It was accomplished using Builder Pattern: the implementation of LabelBuilder trait adds two methods to a Chart, namely x_label_format() and y_label_format(), which allow to specify the format of the corresponding label.

The enum of label formats is:

/// Specifies label format.
/// Default value is `LabelFormat::Value`.
pub enum LabelFormat {
    /// Label is not displayed.
    None,
    /// Label is shown as a value.
    Value,
    /// Label is shown as a custom string.
    Custom(Box<dyn Fn(f32) -> String>),
}

This pull also changes height limit to 3. This will allow user to build spark-lines in the future.

I also changed the order of axis/graph rendering so that axis are rendered first.

The inspiration for the pull request was taken from here.