Luka - Have you tried using Forecast option (works for charts) in the Axes tab. Here is the definition from the documentation.

Forecast

By checking this box and entering an integer in the edit

box, you force the x-axis to be extended to the right of the

actual data points plotted. This is useful when using trendlines.

The forecast line will be dotted.

Hope this helps!

Cheers - DV