This allows writing unit tests without having to call set_style on the
expected buffer.
E.g.:
```rust
use crate::style::Stylize;
let mut buf = Buffer::empty(Rect::new(0, 0, 10, 10));
buf.set_string(0, 0, "foo", Style::new().red());
buf.set_string(0, 1, "bar", Style::new().blue());
assert_eq!(buf, Buffer::with_lines(vec!["foo".red(), "bar".blue()]));
```
Inspired by https://github.com/ratatui-org/ratatui/issues/493#issuecomment-1714844468
A bug in the rendering caused the top line of the chart to be rendered
using the style of the chart, instead of the dataset style. This is
fixed by only setting the style for the width of the text, and not the
entire row.
Fixes: https://github.com/ratatui-org/ratatui/issues/379
Before this change, the style of the points drawn in the graph area could reused to draw the
title of the axis and the legend. Now the style of these components put on top of the graph area
is solely based on the widget style.
