added example to show some colormaps

- this example was copy-pasted from the full_palette example and adapted

Author: jonaspleyer

plotters/examples/colormaps.rs

@@ -0,0 +1,68 @@
+use plotters::prelude::*;
+
+const OUT_FILE_NAME: &'static str = "plotters-doc-data/colormaps.png";
+
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let colormaps_rgb: [(Box<dyn ColorMap<RGBColor>>, &str); 4] = [
+        (Box::new(ViridisRGB {}),"Viridis"),
+        (Box::new(BlackWhite {}),"BlackWhite"),
+        (Box::new(Bone {}),"Bone"),
+        (Box::new(Copper {}),"Copper"),
+    ];
+
+    let colormaps_hsl: [(Box<dyn ColorMap<HSLColor>>, &str); 2] = [
+        (Box::new(MandelbrotHSL {}), "MandelbrotHSL"),
+        (Box::new(VulcanoHSL {}), "VulcanoHSL"),
+    ];
+
+    let size_x: i32 = 800;
+    let n_colormaps = colormaps_rgb.len() + colormaps_hsl.len();
+    let size_y = 200 + n_colormaps as u32 * 100;
+    let root = BitMapBackend::new(OUT_FILE_NAME, (size_x as u32, size_y)).into_drawing_area();
+
+    root.fill(&WHITE)?;
+
+    let mut chart = ChartBuilder::on(&root)
+        .caption("Demonstration of predefined colormaps", ("sans-serif", 20))
+        .build_cartesian_2d(-150.0..size_x as f32+50.0, 0.0..3.0*(n_colormaps as f32))?;
+
+    use plotters::style::text_anchor::*;
+    let centered = Pos::new(HPos::Center, VPos::Center);
+    let label_style = TextStyle::from(("monospace", 14.0).into_font()).pos(centered);
+
+    let mut colormap_counter = 0;
+    macro_rules! plot_colormaps(
+        ($colormap:expr) => {
+            for (colormap, colormap_name) in $colormap.iter() {
+                chart.draw_series(
+                    (0..size_x as i32).map(|x| {
+                        Rectangle::new([
+                            (x as f32,     3.0*(n_colormaps - 1 - colormap_counter) as f32 + 0.5),
+                            (x as f32+1.0, 3.0*(n_colormaps - 1 - colormap_counter) as f32 + 2.5)
+                        ],
+                    colormap.get_color_normalized(x as f32, 0.0, size_x as f32).filled())
+                    })
+                )?;
+                chart.draw_series(
+                    [Text::new(colormap_name.to_owned(), (-75.0, 3.0*(n_colormaps-1-colormap_counter) as f32 + 1.5), &label_style)]
+                )?;
+                colormap_counter+=1;
+            }
+        }
+    );
+
+    plot_colormaps!(colormaps_rgb);
+    plot_colormaps!(colormaps_hsl);
+
+
+    // To avoid the IO failure being ignored silently, we manually call the present function
+    root.present().expect("Unable to write result to file, please make sure 'plotters-doc-data' dir exists under current dir");
+    println!("Result has been saved to {}", OUT_FILE_NAME);
+
+    Ok(())
+}
+#[test]
+fn entry_point() {
+    main().unwrap()
+}