diff --git a/examples/3d-plot2.rs b/examples/3d-plot2.rs
index f2704658..38800759 100644
--- a/examples/3d-plot2.rs
+++ b/examples/3d-plot2.rs
@@ -1,11 +1,20 @@
 use plotters::prelude::*;
+use std::f64::consts::PI;
 fn pdf(x: f64, y: f64) -> f64 {
-    const SDX: f64 = 0.1;
-    const SDY: f64 = 0.1;
-    const A: f64 = 5.0;
-    let x = x as f64 / 10.0;
-    let y = y as f64 / 10.0;
-    A * (-x * x / 2.0 / SDX / SDX - y * y / 2.0 / SDY / SDY).exp()
+    // see https://mathworld.wolfram.com/BivariateNormalDistribution.html
+    // assumes x_bar = 0 and y_bar = 0
+    const SDX: f64 = 0.3;
+    const SDY: f64 = 0.3;
+    const vx: f64 = SDX * SDX;
+    const vy : f64 = SDY * SDY;
+    const sdx_sdy: f64 = SDX * SDY;
+    const RHO : f64 = 0.9;
+    const c1: f64 = 1.0 - RHO * RHO;
+    let c2 : f64 = c1.sqrt();
+    let A: f64 = 1.0 / (2.0 * PI * sdx_sdy * c2);
+    let Z: f64 = x * x / vx - 2.0 * RHO * x * y / sdx_sdy  + y * y / vy; 
+    let res = A * (-Z / 2.0 / c1).exp();
+    res
 }
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
@@ -17,7 +26,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
         let mut chart = ChartBuilder::on(&root)
             .caption("2D Guassian PDF", ("sans-serif", 20))
-            .build_cartesian_3d(-3.0..3.0, 0.0..6.0, -3.0..3.0)?;
+            .build_cartesian_3d(-1.0..1.0, 0.0..3.0, -1.0..1.0)?;
         chart.with_projection(|mut p| {
             p.pitch = 1.57 - (1.57 - pitch as f64 / 50.0).abs();
             p.scale = 0.7;
@@ -28,8 +37,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
         chart.draw_series(
             SurfaceSeries::xoz(
-                (-15..=15).map(|x| x as f64 / 5.0),
-                (-15..=15).map(|x| x as f64 / 5.0),
+                (-30..=30).map(|x| x as f64 / 30.0),
+                (-30..=30).map(|x| x as f64 / 30.0),
                 pdf,
             )
             .style_func(&|&v| {