[v2] Last vertical line is darker than oters

[Seb33300]

Hi

As you can see when looking on samples pages:
https://github.com/nnnick/Chart.js/blob/v2.0-dev/samples/bar-multi-axis.html
https://github.com/nnnick/Chart.js/blob/v2.0-dev/samples/bar-stacked.html
https://github.com/nnnick/Chart.js/blob/v2.0-dev/samples/bar.html
...

The last vertical separation bar (between June and July) is always darker then others.
It is the same colors as the 0 axis.
I also have this issue on my owns charts.

Is it a bug?

[etimberg]

Fixed in #1915