Duplicate axis tick labels disappear

[dnschnur]

From Google Groups, when two axis tick labels in the same axis have the same text, only one of them appears.

This was a totally predictable consequence of cutting a corner in the canvas-text implementation for 0.8, and unfortunately I made the mistake of thinking that it wouldn't arise in normal usage. I didn't think carefully enough about the fact that duplicate ticks are pretty common when using the time plugin.