Hi Fabrizio,
If you put 50 in the 'bounce rate greater than box' instead of 0.5 then the table shown below the graph shows the data you want (only bounce rates over 50%, I think that is what you are after right?). I guess the graph cannot be show this filtered data although you can click the 'select a metric' link next to the visits dropdown on the top/left of the graph and add bounce rate to see the average bounce rate by visits/day if that helps with getting a baseline.