Custom hover labels for plot x/y values

[isegal]

Currently, it is possible to customize the name of the line graph, but x and y coordinate names for hovering over the plot is not customizable. When hovering over a plot line, the user will always see

<name>
x = 123
y = 234

This is a request to add a set of api functions to be able to customize how the hover text gets displayd in the plot graph.

For example, the user may want to label x axis as time and y axis as percent or cost or another metric.

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

One option is to allow the user to add a custom closure that takes the plot name and Value as input and returns a formatted string of how the values are to be displayed.

Plot::new("my_plot")
    .custom_label_func(|name, value| {
        if let Some(name) = name {
            format!("{}: {:.*}%", name, 1, value.y).to_string()
        } else {
            "CPU".to_string()
        }
    })
    .show(ui, |plot_ui| plot_ui.line(line));

This would allow relabeling x and y and display a custom formatted string for the hover values.

Describe alternatives you've considered
Considered adding x and y names, but this removes some flexibility on how the hover would be displayd.