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.

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)>>();