Transact SQL :: Advanced Query With Windows Function?

SQL Sever 2012, I have a table where I store data related with a Log, the columns of the table are three, IDTable, DateLog and CountLog, the column CountLog is a cumulative value, each new rows can increment this value, my query will receive two parameters related with the Date,  @dateFrom and @dateTo, with this information is necesary to get the diff between two months, for example:

CREATE TABLE MyTable
(
  IDTable INT IDENTITY NOT NULL,
  DateLog  DATE NOT NULL,
  CountLog INT NOT NULL
)

[code]....

In this case I need to apply a filter with the Dates:

@dateFrom ='2014-01-01',
@dateTo     ='2014-04-28'

But according with this filter I need to limit the results only for this period between the date, but I need that the first month (January 2014) take the information of the max value from previous month even not included in the filter, in this case the results will be the next:

Id        MonthYear      Diff
1            Jan2014       5
2            Feb2014       3
3            March2014   0
4            April2014      5

With the results, for the first mont (January) is, according to the range of times, January is the lower limit,the last cumulative value of CountLog before the first month of the range  was 2 (December 2013), the max value of the CountLog for this first month was 7, the diff is 7-2 equal to 5 , in the case of the next month (February) the max value of the CountLog was 10, the diff is 10 - 7 equal to 3 and this way.  

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