fixing code to include both end points in n samples for better symmet…

…ry and span

src/traces/violin/calc.js

@@ -39,7 +39,7 @@ module.exports = function calc(gd, trace) {
             // step that well covers the bandwidth and is multiple of span distance
             var dist = span[1] - span[0];
             var n = Math.ceil(dist / (bandwidth / 3));
-            var step = dist / n;
+            var step = (n > 1) ? dist / (n - 1) : 0;
 
             if(!isFinite(step) || !isFinite(n)) {
                 Lib.error('Something went wrong with computing the violin span');