Fix misalignment of line chart ticks with x axis dates

When plotting a time series there's a misalignment of plotted line ticks with the x-axis time scale. 
For example take look at the image below. The x-axis are dates, y-axis are prices:
![misaligned values](http://i.imgur.com/lDM8hkD.png?1)
You can see that the tooltip indicates the date to be 20, and the tick is not aligned with x-axis. 
This added line fixes this error for me. So you can see the same graph aligned in the following picture: 
![aligned values](http://i.imgur.com/WAORTn7.png?1)

I don't know how it will behave when plotting non-time series.