Now I do not understand way to make forecast model in my case.
There is data includes money amount per month for 2-year period (its synthetic data, period in input data may vary).
The requested result of the model is getting forecast chart. Forecast period is 1 year, and chart must be calculated on a basis of trend line (y=bx+a) and average money amount per months for vary period (1 year, 2 year, 1 year 2 months etc.).
I made chart calculating trend line (get a and b coefficients). But dimension of this chart is real months for each year. When I trying change dimension to month (to calculate average values of amount) - trend line coefficients changed. As result, my chart is work correctly for 1 basis year. When more than 1 year is selected, trend line calculating is wrong.