add end-to-end test as an example on thick lines

this code previously failed getting stuck in a very very long loop.
the example shows the pathological case (separately included as a unit test) but also some simple cases with with and without capping.

Author: el-hult

plotters/examples/thick_lines.rs

@@ -0,0 +1,27 @@
+use plotters::prelude::*;
+
+pub fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let root = BitMapBackend::new("test.bmp", (1000, 1000)).into_drawing_area();
+    let mut chart = ChartBuilder::on(&root)
+        .build_cartesian_2d(0.0..1000.0, 0.0..1000.0)?;
+    let pahts = vec![
+        PathElement::new(
+            vec![(336.0, 614.0), (339.0, 674.0),(341.0,714.0)],
+            ShapeStyle::from(RED).stroke_width(2),
+        ),
+        PathElement::new(
+            vec![(100.0, 100.0), (150.0, 150.0),(200.0,100.0)],
+            ShapeStyle::from(BLUE).stroke_width(2),
+        ),
+        PathElement::new(
+            vec![(400.0, 400.0), (400.0, 450.0),(400.0,500.0)],
+            ShapeStyle::from(GREEN).stroke_width(5),
+        ),
+        PathElement::new(
+            vec![(900.0, 410.0), (600.0, 400.0),(900.0,500.0), (1000.0, 1000.0)],
+            ShapeStyle::from(YELLOW).stroke_width(10),
+        ),
+    ];
+    chart.draw_series(pahts.into_iter())?;
+    Ok(())
+}