Add single series forecasting from a multiseries line chart

[DenielPereira]

Summary

Add a single series forecasting (dashed line) from a multiseries line chart, like indicated in the image:

API Changes

I think changing the forecastDataPoints option, somehow informing that the predictive data should be in the last line of the graph.

Intended Use Case

If the line graph is showing time (year and month), the predictive values ​​would only be in the last line, because the other lines are past data.