forecasting sales based on history

Hi MagikJukas 

First of all I used a Date table for these measures and converted your Day, Month & Year columns in Sales table to a single date field.  So, the model looks like this:

Now, here's a basic Sales amount measure and a SalesMTD measure to use in a line chart:

Sales Amount = SUM(Sales[Sales])

Sales MTD = 
VAR _MaxDateInData = MAXX(ALL(Sales), Sales[Date])
VAR _Result = IF(MAX('Date'[Date]) <= _MaxDateInData, TOTALMTD([Sales Amount], 'Date'[Date]) )
RETURN
    _Result

 I can set up a line chart like this to track MTD sales trajectories across years.  Note, the Year, Month & Day columns are from the date table:

 Lastly, the forecast measure.  There are many different rules/logic you could use to forecast sales.  This one carries on from whatever the actual sales are this month and adds the average of the same day in previous years, and returns a cumulative amount like the Sales MTD measure

Sales Forecast MTD = 
VAR _MaxDateInData = MAXX(ALL(Sales), Sales[Date])
VAR _CurrentDate = MAX('Date'[Date])
VAR _SalesMTD = TOTALMTD([Sales Amount], 'Date'[Date])
VAR _ForecastMTD = 
SUMX(
    DATESBETWEEN('Date'[Date], _MaxDateInData + 1, _CurrentDate),
    VAR _ThisDate = CALCULATE(MAX('Date'[Date]))
    RETURN
    AVERAGEX(
        FILTER(ALL('Date'), 'Date'[Month Num] = MONTH(_ThisDate) && 'Date'[Day] = DAY(_ThisDate) && 'Date'[Year] < YEAR(_ThisDate)), 
        [Sales Amount]
    )
)
VAR _Result = IF(MAX('Date'[Date]) >= _MaxDateInData, _SalesMTD + _ForecastMTD )
RETURN
    _Result

 Here's what that can look like on a line chart, along with Sales MTD.

Frustratingly it doesn't look like you can combine the line charts.  ie have Sales MTD split by Year, plus the Sales Forecast MTD all in one line chart.  Perhaps you can achieve that by layering 2 line charts on top of each other or delve into creating a custom chart with Deneb if you want that.