yaa, Google Dance is term that describes Google's updating process of SERPs that shuffles or shifts web page placements.
It could be a number of different things, this chart is not enough on its own to make a conclusion. There is no way to avoid the google dance, only authority sites appear to keep stable rankings.
Yes it may be google dance or the some of the top keywords was in the rank. So the traffic increased.
I doubt that the chart refelect the Google Dance since the dance is based on Page Rank calculations. What you are looking at is the traffic to your site and if you look closer at your "visitors", then you would see that it is not Google. As for the dance it's self, don't worry about it and don't waste your time even trying to understand it.
oh, the traffic is only from Google, this screen print is capture when I check report for search engine google only. thanks for your reply.
google dance is where you change from 30th page to 1st page and back. This happens alot when google is updating