fix(widgets/barchart): fix chart filled more than actual (#383)

* Fix barchart incorrectly showing chart filled more than actual

Determination of how filled the bar should be was incorrectly taking the
entire chart height into account, when in fact it should take height-1, because
of extra line with label. Because of that the chart appeared fuller than it
should and was full before reaching maximum value.

* Add a test case for checking if barchart fills correctly up to max value

Co-authored-by: Kemyt <kemyt4@gmail.com>

src/widgets/barchart.rs

@@ -157,7 +157,7 @@ impl<'a> Widget for BarChart<'a> {
             .map(|&(l, v)| {
                 (
                     l,
-                    v * u64::from(chart_area.height) * 8 / std::cmp::max(max, 1),
+                    v * u64::from(chart_area.height - 1) * 8 / std::cmp::max(max, 1),
                 )
             })
             .collect::<Vec<(&str, u64)>>();

tests/widgets_barchart.rs

@@ -0,0 +1,40 @@
+use tui::backend::TestBackend;
+use tui::buffer::Buffer;
+use tui::widgets::{BarChart, Block, Borders};
+use tui::Terminal;
+
+#[test]
+fn widgets_barchart_not_full_below_max_value() {
+    let test_case = |expected| {
+        let backend = TestBackend::new(30, 10);
+        let mut terminal = Terminal::new(backend).unwrap();
+
+        terminal
+            .draw(|f| {
+                let size = f.size();
+                let barchart = BarChart::default()
+                    .block(Block::default().borders(Borders::ALL))
+                    .data(&[("empty", 0), ("half", 50), ("almost", 99), ("full", 100)])
+                    .max(100)
+                    .bar_width(7)
+                    .bar_gap(0);
+                f.render_widget(barchart, size);
+            })
+            .unwrap();
+        terminal.backend().assert_buffer(&expected);
+    };
+
+    // check that bars fill up correctly up to max value
+    test_case(Buffer::with_lines(vec![
+        "┌────────────────────────────┐",
+        "│              ▇▇▇▇▇▇▇███████│",
+        "│              ██████████████│",
+        "│              ██████████████│",
+        "│       ▄▄▄▄▄▄▄██████████████│",
+        "│       █████████████████████│",
+        "│       █████████████████████│",
+        "│       ██50█████99█████100██│",
+        "│empty  half   almost full   │",
+        "└────────────────────────────┘",
+    ]));
+}