fix(barchart): empty groups causes panic (#333)

This unlikely to happen, since nobody wants to add an empty group.
Even we fix the panic, things will not render correctly.
So it is better to just not add them to the BarChart.

Signed-off-by: Ben Fekih, Hichem <hichem.f@live.de>

src/widgets/barchart/mod.rs

@@ -87,7 +87,10 @@ impl<'a> BarChart<'a> {
     ///        .data(BarGroup::default().bars(&[Bar::default().value(10), Bar::default().value(20)]));
     /// ```
     pub fn data(mut self, data: impl Into<BarGroup<'a>>) -> BarChart<'a> {
-        self.data.push(data.into());
+        let group: BarGroup = data.into();
+        if !group.bars.is_empty() {
+            self.data.push(group);
+        }
         self
     }
 
@@ -737,4 +740,20 @@ mod tests {
                 .add_modifier(Modifier::BOLD)
         )
     }
+
+    #[test]
+    fn test_empty_group() {
+        let chart = BarChart::default()
+            .data(BarGroup::default().label("invisible".into()))
+            .data(
+                BarGroup::default()
+                    .label("G".into())
+                    .bars(&[Bar::default().value(1), Bar::default().value(2)]),
+            );
+
+        let mut buffer = Buffer::empty(Rect::new(0, 0, 3, 3));
+        chart.render(buffer.area, &mut buffer);
+        let expected = Buffer::with_lines(vec!["  █", "█ █", " G "]);
+        assert_buffer_eq!(buffer, expected);
+    }
 }