thanks a lot for your reply!
Based on your sample, which did indeed work for me as well, I was able to narrow the cause down further. Apparently it's got to do with having data points that are just a minute apart. (This was the case in my original data, even though the axis configuration implied something different.)
If you simply change the "AddYears" call in line 86-98 to "AddMinutes", you will see the bugged behaviour:
I've also built an extended version of your chart (source code attached) that visualizes the difference between using years and using minutes. It turns out like this:
(I made the year-spaced series to go 2 instead of 1 for better visibility.)