It's a bit strange comparing Chrome's releases with the others. Firefox had many more pestering minor updates in that time, which are comparable to Chrome's, only Firefox labeled them differently. Safari also had major changes between version numbers.
To get an accurate picture the diffs between updates should be compared, Chrome made version numbers meaningless.
Firefox's minor updates were not really feature updates.
On the chart Chrome whole number releases are new feature releases. Firefox 3.5 and 3.6 are seperate because they are similar feature releases. 3.6.0 to 3.6.1 are not.
To get an accurate picture the diffs between updates should be compared, Chrome made version numbers meaningless.