fix: fix handling of multi-byte chars in bar chart (#1934)

The split_at method requires that the split point is at a valid utf8
character boundary.

Fixes: https://github.com/ratatui/ratatui/issues/1928

ratatui-widgets/src/barchart.rs

@@ -1441,4 +1441,35 @@ mod tests {
         assert_eq!(updated_chart.data.len(), 2);
         assert_eq!(updated_chart.data[1].bars, [Bar::with_label("Blue", 3)]);
     }
+
+    /// Regression test for issue <https://github.com/ratatui/ratatui/issues/1928>
+    ///
+    /// This test ensures that the `BarChart` doesn't panic when rendering text labels with
+    /// multi-byte characters in the bar labels.
+    #[test]
+    fn regression_1928() {
+        let text_value = "\u{202f}"; // Narrow No-Break Space
+        let bars = [
+            Bar::default().text_value(text_value).value(0),
+            Bar::default().text_value(text_value).value(1),
+            Bar::default().text_value(text_value).value(2),
+            Bar::default().text_value(text_value).value(3),
+            Bar::default().text_value(text_value).value(4),
+        ];
+        let chart = BarChart::default()
+            .data(BarGroup::default().bars(&bars))
+            .bar_gap(0)
+            .direction(Direction::Horizontal);
+        let mut buffer = Buffer::empty(Rect::new(0, 0, 4, 5));
+        chart.render(buffer.area, &mut buffer);
+        #[rustfmt::skip]
+        let expected = Buffer::with_lines([
+            "\u{202f}   ",
+            "\u{202f}   ",
+            "\u{202f}█  ",
+            "\u{202f}██ ",
+            "\u{202f}███",
+        ]);
+        assert_eq!(buffer, expected);
+    }
 }

ratatui-widgets/src/barchart/bar.rs

@@ -216,6 +216,13 @@ impl<'a> Bar<'a> {
             buf.set_stringn(area.x, area.y, text, bar_length, style);
             // render the second part with the bar_style
             if text.len() > bar_length {
+                // Find the last character boundary at or before bar_length
+                let bar_length = text
+                    .char_indices()
+                    .take_while(|(i, _)| *i < bar_length)
+                    .last()
+                    .map_or(0, |(i, c)| i + c.len_utf8());
+
                 let (first, second) = text.split_at(bar_length);
 
                 let style = bar_style.patch(self.style);