#10724 Fix finding the x bucket

public/app/plugins/panel/heatmap/heatmap_tooltip.ts

@@ -153,7 +153,15 @@ export class HeatmapTooltip {
 
   getXBucketIndex(offsetX, data) {
     let x = this.scope.xScale.invert(offsetX - this.scope.yAxisWidth).valueOf();
-    let xBucketIndex = getValueBucketBound(x, data.xBucketSize, 1);
+    let xBucket = _.find(data.buckets, (bucket, bucketIndex) => {
+      return x-bucket.x <= data.xBucketSize && x-bucket.x >0;
+    });
+    let xBucketIndex;
+    if(!xBucket)
+      xBucketIndex = getValueBucketBound(x, data.xBucketSize, 1);
+    else
+      xBucketIndex = xBucket.x;
+
     return xBucketIndex;
   }