Data forecast

Hello, I'm not exactly sure if this is the right forum, or even if what I'm looking for is actually called data-forecasting, but here's my question anyways:

I have a set of data-values. The data shows the price of an item, on a given day, so consider for instance 180 days, with 180 prices - which can form a graph.

How can you create a forecast for the graph, mathematically and without 3rd party CVS-programs like Excel? I'm not interested in smoothing or taking trends, seasons or something like that into account - but merely, continue the graph with some 'predicted' values.

I know there may not be just one solution, but any guidance is much appreciated.
Thanks in advance.