Rolling more than 12 months

[PepeMontes]

Hello everyone,
I'm trying to show the last 20 months in bar chart, i tired many ways and no one gives me the result that im looking for.
First i tried with the next expression: sum({<idmont={'>=$(=max(idmont)-20)<=$(=max(idmont))'}>}CantidadVen).
This should suppose to be ok, even in the preview of this expression i can see that the range of dates are ok.

But nothing it's displayed into the bar chart.

This is my dates section into the script:

Fechas:
LOAD

Date(FechaVenta) AS FechaVenta,

Year(FechaVenta) AS Year,

'Q' & Ceil(Month(FechaVenta) / 3) AS Quarter,

Month(FechaVenta) As Month,

Day(FechaVenta) As Day,


Year((FechaVenta)-1)*12 + Num(Month(FechaVenta)) as idmont,

Week(FechaVenta) As Week;

Load Date(MinDate + IterNo() -1 ) AS FechaVenta While (MinDate + IterNo() - 1) <= Num(MaxDate);

Load

Min(FechaVenta) AS MinDate,

Max(FechaVenta) AS MaxDate

RESIDENT Documentos;

Someone did something like this before?

Regards.