fix(violin): prevent FP drift in KDE sampling loop (avoid underrun/overrun on tiny spans)

draftlogs/7581_fix.md

@@ -0,0 +1 @@
+- Fix KDE sampling precision in violin trace to eliminate floating-point drift and prevent density underrun/overrun ([#7581](https://github.com/plotly/plotly.js/pull/7581))

src/traces/violin/calc.js

@@ -48,9 +48,11 @@ module.exports = function calc(gd, trace) {
             }
 
             var kde = helpers.makeKDE(cdi, trace, vals);
-            cdi.density = new Array(n);
+            // n intervals means n + 1 sample points to include both endpoints
+            cdi.density = new Array(n + 1);
 
-            for(var k = 0, t = span[0]; t < (span[1] + step / 2); k++, t += step) {
+            for(var k = 0; k <= n; k++) {
+                var t = span[0] + k * step;
                 var v = kde(t);
                 cdi.density[k] = {v: v, t: t};
                 maxKDE = Math.max(maxKDE, v);

test/jasmine/tests/violin_test.js

@@ -401,6 +401,20 @@ describe('Test violin calc:', function() {
         expect(cd.length).toBe(1, '# of violins');
         expect(cd[0].bandwidth).toBe(0, 'bandwidth');
     });
+
+    it('should produce exactly n + 1 density samples for tiny or near-equal spans', function() {
+        var cd = _calc({
+            type: 'violin',
+            x: [0, 0],
+            y: [0.5006312999999999, 0.5006313]
+        });
+        var cdi = cd[0];
+
+        var dist = cdi.span[1] - cdi.span[0];
+        var n = Math.ceil(dist / (cdi.bandwidth / 3));
+
+        expect(cdi.density.length).toBe(n + 1);
+    });
 });
 
 describe('Test violin hover:', function() {