violin: handle data with zero standard deviation

Author: antoinerg

src/traces/violin/calc.js

@@ -100,8 +100,15 @@ function silvermanRule(len, ssd, iqr) {
 function calcBandwidth(trace, cdi, vals) {
     var span = cdi.max - cdi.min;
 
-    // plot single-value violin with bandwidth of 1
-    if(!span) return 1;
+    // If span is zero
+    if(!span) {
+        if(trace.bandwidth) {
+            return trace.bandwidth;
+        } else {
+            // plot single-value violin with bandwidth of 1
+            return 1;
+        }
+    }
 
     // Limit how small the bandwidth can be.
     //

test/image/baselines/violin_only_zeroes.png

@@ -0,0 +1,23 @@
+{
+  "data": [{
+    "type": "violin",
+    "y": [0, 0, 0, 0],
+    "points": "all",
+    "pointpos": 0,
+    "bandwidth": 0.025,
+    "meanline": {
+      "visible": true
+    }
+  }, {
+    "type": "violin",
+    "y": [2, 2, 2, 2],
+    "points": "all",
+    "bandwidth": 0.1,
+    "pointpos": 0
+  }, {
+    "type": "violin",
+    "y": [0, 1, 1.5, 2],
+    "points": "all",
+    "pointpos": 0
+  }]
+}