fix(chart): use graph style for top line (#462)

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

src/widgets/chart.rs

@@ -560,7 +560,7 @@ impl<'a> Widget for Chart<'a> {
 
         if let Some((x, y)) = layout.title_x {
             let title = self.x_axis.title.unwrap();
-            let width = graph_area.right().saturating_sub(x);
+            let width = title.width() as u16;
             buf.set_style(
                 Rect {
                     x,
@@ -575,7 +575,7 @@ impl<'a> Widget for Chart<'a> {
 
         if let Some((x, y)) = layout.title_y {
             let title = self.y_axis.title.unwrap();
-            let width = graph_area.right().saturating_sub(x);
+            let width = title.width() as u16;
             buf.set_style(
                 Rect {
                     x,
@@ -718,4 +718,15 @@ mod tests {
         assert_eq!("Line".parse::<GraphType>(), Ok(GraphType::Line));
         assert_eq!("".parse::<GraphType>(), Err(ParseError::VariantNotFound));
     }
+
+    #[test]
+    fn it_does_not_panic_if_title_is_wider_than_buffer() {
+        let widget = Chart::default()
+            .y_axis(Axis::default().title("xxxxxxxxxxxxxxxx"))
+            .x_axis(Axis::default().title("xxxxxxxxxxxxxxxx"));
+        let mut buffer = Buffer::empty(Rect::new(0, 0, 8, 4));
+        widget.render(buffer.area, &mut buffer);
+
+        assert_eq!(buffer, Buffer::with_lines(vec![" ".repeat(8); 4]))
+    }
 }

tests/widgets_chart.rs

@@ -618,3 +618,41 @@ fn widgets_chart_can_have_a_legend() {
 
     terminal.backend().assert_buffer(&expected);
 }
+
+#[test]
+fn widgets_chart_top_line_styling_is_correct() {
+    let backend = TestBackend::new(9, 5);
+    let mut terminal = Terminal::new(backend).unwrap();
+
+    let title_style = Style::default().fg(Color::Red).bg(Color::LightRed);
+    let data_style = Style::default().fg(Color::Blue);
+
+    terminal
+        .draw(|f| {
+            let data: [(f64, f64); 2] = [(0.0, 1.0), (1.0, 1.0)];
+            let widget = Chart::new(vec![Dataset::default()
+                .data(&data)
+                .graph_type(ratatui::widgets::GraphType::Line)
+                .style(data_style)])
+            .y_axis(
+                Axis::default()
+                    .title(Span::styled("abc", title_style))
+                    .bounds([0.0, 1.0])
+                    .labels(create_labels(&["a", "b"])),
+            )
+            .x_axis(Axis::default().bounds([0.0, 1.0]));
+            f.render_widget(widget, f.size());
+        })
+        .unwrap();
+
+    let mut expected = Buffer::with_lines(vec![
+        "b│abc••••",
+        " │       ",
+        " │       ",
+        " │       ",
+        "a│       ",
+    ]);
+    expected.set_style(Rect::new(2, 0, 3, 1), title_style);
+    expected.set_style(Rect::new(5, 0, 4, 1), data_style);
+    terminal.backend().assert_buffer(&expected);
+}