diff --git a/README.md b/README.md
index 139740f..e0eb2d6 100644
--- a/README.md
+++ b/README.md
@@ -52,14 +52,25 @@ is therefore often considered to be of "linear" complexity in practice,
 although it is in the worst case superpolynomial when performed until
 convergence.
 
+## Options
+
 You can greatly reduce the running time by adjusting the required delta
-threshold. The following code executes the algorithm until less than 5% of the
-data points shifted their cluster assignment in the last iteration:
+threshold. With the following options the algorithm finishes when less than 5%
+of the data points shifted their cluster assignment in the last iteration:
 
 ```go
-km, _ := kmeans.NewWithOptions(0.05, false)
+km, err := kmeans.NewWithOptions(0.05, false)
 ```
 
+If you are working with two-dimensional data sets, kmeans can generate
+beautiful graphs (like the one above) for each iteration of the algorithm:
+
+```go
+km, err := kmeans.NewWithOptions(0.01, true)
+```
+
+Careful: this will generate PNGs in your current working directory.
+
 ## Development
 
 [![GoDoc](https://godoc.org/github.com/golang/gddo?status.svg)](https://godoc.org/github.com/muesli/kmeans)
diff --git a/cluster.go b/cluster.go
index 13a019c..4f9fb0d 100644
--- a/cluster.go
+++ b/cluster.go
@@ -26,6 +26,7 @@ func (c Clusters) recenter() {
 	}
 }
 
+// reset clears all point assignments
 func (c Clusters) reset() {
 	for i := 0; i < len(c); i++ {
 		c[i].Points = Points{}