The simple answer is like this. I used Unique count of clientip and a Date Histogram with a daily interval.
But the problem with this is that it's counting how many unique IPs there are each day. So the count on the second day includes all the IP addresses on that day even if they also were visitors on the previous day.
There are several other posts on this topic but I don't think any of them have a good solutions you could implement in Kibana. For example;
Here's a post that looks like it's developing the correct query in Elasticsearch but I'm not sure if we can do that in Kibana;
I'll try to spend some time to see if Kibana can do this;