Forecast and Backcast are related to trendlines. Referring to the HELP:
Trendlines In selected QlikView charts expression plots can be complemented or replaced by statistical trend lines. Trend lines can only be displayed in scatter charts, line charts and in bar/combo charts with maximally one dimension and one expression shown as bars. For other types of charts, the settings in the Trendlines group are unavailable and have no effect. In scatter charts the data points are treated as if y=f(x). For bar, line and combo charts it is allowed to deselect all options under Display Options and still add trend lines, which will then be plotted without the underlying data points. Trend lines in bar, line and combo charts may be extrapolated by specifying a forecast and/or backcast interval (Axes page). The extrapolated lines will be dotted. Trend lines in charts with a discrete x-axis will be shown as lines with symbols. On a continuous axis only the line will be shown.
The average is plotted as a straight line.
A linear regression line is plotted.
Polynomial of 2nd degree
A polynomial trend line of the second degree is plotted.
Polynomial of 3rd degree
A polynomial trend line of the third degree is plotted.
Polynomial of 4th degree
A polynomial trend line of the fourth degree is plotted.
An exponential trend line is plotted.
If this check box is marked for a specific expression, the expression’s trend lines will be complemented by the trendline equation expressed as text in the chart.
If this check box is marked for a specific expression, the expression’s trend lines will be complemented by the coefficient of determination expressed as text in the chart.
This function operates on trend lines. Enter into the text edit box how far back you would like to estimate the trend line. See Chart Properties: Expressions. The backcast portion of trend lines is displayed as dotted.
Enter into the text edit box how far you wish to predict the trend line. See Chart Properties: Expressions. The forecast portion of trend lines is displayed as dotted.
So try creating a simple line chart (best with numerical axis), add any trendline (start maybe with a simple one, linerar, and play around with the options in axis tab).
Hope this helps,