Add first transformed Polygon

@@ -50,8 +50,7 @@ impl Coordinate {
             let Coordinate(x, y) = v[0];
 
             if x != bx || y != by {
-                let doinc = (2*err >= dy, 2*err <= dx);
-                let (x, y, err) = match doinc {
+                let (x, y, err) = match (2*err >= dy, 2*err <= dx) {
                     (true, false) => (x + sx,      y, err + dy),
                     (false, true) => (     x, y + sy, err + dx),
                     _             => (x + sx, y + sy, err + dx + dy ),
@@ -218,7 +217,7 @@ impl Drawable for Polygon {
                     r.append(&mut i.line(j)[1..].to_vec());
                     i = *j;
                 }
-                let mut j = i.line(&a);
+                let mut j = a.line(&i);
                 let     l = j.len();
                 r.append(&mut j[1..l-1].to_vec());
                 Coordinates(r)

@@ -254,9 +254,50 @@ fn _line() {
 
     println!();
     let pg = Polygon(
-        Coordinates(vec!( Coordinate(  0, -20)
-                        , Coordinate( 20,  20)
-                        , Coordinate(-20,  20) )));
+        Coordinates(vec!( Coordinate(  0, -10)
+                        , Coordinate( 10,  10)
+                        , Coordinate(-10,  10) )));
+    println!("{:>14} : {}", pg, pg.plot());
+
+    let i = Vector(Fractional(  0,1), Fractional(-30,1), Fractional(0,1));
+    let j = Vector(Fractional( 30,1), Fractional( 30,1), Fractional(0,1));
+    let k = Vector(Fractional(-30,1), Fractional( 30,1), Fractional(0,1));
+
+    let rot :TMatrix<Fractional> = rotate_z(20);
+    let Vector(ix, iy, _) = rot.apply(&i);
+    let Vector(jx, jy, _) = rot.apply(&j);
+    let Vector(kx, ky, _) = rot.apply(&k);
+
+    fn to_i32(x :Fractional) -> i32 {
+        let Fractional(n, d) = x;
+        (n / d + if (n % d).abs() < (n / 2).abs() { 0 } else { 1 }) as i32
+    }
+
+    println!();
+    let pg = Polygon(
+        Coordinates(vec!( Coordinate(to_i32(ix) + 100, to_i32(iy) + 100)
+                        , Coordinate(to_i32(jx) + 100, to_i32(jy) + 100)
+                        , Coordinate(to_i32(kx) + 100, to_i32(ky) + 100) )));
+    println!("{:>14} : {}", pg, pg.plot());
+
+    let i = Vector(  0.0, -30.0, 0.0);
+    let j = Vector( 30.0,  30.0, 0.0);
+    let k = Vector(-30.0,  30.0, 0.0);
+
+    let rot :TMatrix<f64> = rotate_z(20);
+    let Vector(ix, iy, _) = rot.apply(&i);
+    let Vector(jx, jy, _) = rot.apply(&j);
+    let Vector(kx, ky, _) = rot.apply(&k);
+
+    fn to_i32_2(x :f64) -> i32 {
+        x.round() as i32
+    }
+
+    println!();
+    let pg = Polygon(
+        Coordinates(vec!( Coordinate(to_i32_2(ix) + 100, to_i32_2(iy) + 100)
+                        , Coordinate(to_i32_2(jx) + 100, to_i32_2(jy) + 100)
+                        , Coordinate(to_i32_2(kx) + 100, to_i32_2(ky) + 100) )));
     println!("{:>14} : {}", pg, pg.plot());
 }