The problem is that quotes are used to represent string literals wherever keywords are allowed (legend doesn’t support keywords). Let me try to figure out a solution.
Though I did see the above mentioned behaviour in chart, am not sure if the issue is with the library or the data provided. If you can create a jsfiddle with the data, that would help me in figuring out the issue.
On further research, there is no bug in the library actually. I had confused it with something else. In your code I hadn’t observed that you have missed interval property of axis. Unless you set interval property, intervalType is ignored.