Let's assume you've a [Time] dimension with [Year], [Month] and [Day] levels.
If
SELECT
[Time].[Jan 2015]:[Time].[Dec 2015] on 0,
[Measures].[Sales] on 1
FROM
[Cube]
Returns the sales for all months of 2015. We can add a calculated measure to get ratio :
WITH
MEMBER [Sales Ratio] AS DivN(
[Sales],
( ParallelPeriod( [Time].[Year], 1, [Time].current ), [Sales] )
SELECT
[Time].[Jan 2015]:[Time].[Dec 2015] on 0,
{[Sales],[Sales Ratio]} on 1
FROM
[Cube]
DivN is icCube specific and allows for dividing being 'empty' safe.
ParallelPeriod is a standard MDX function, that returns previous years month. You could also use Lag(-12), that is 'travelling' backwards in a level 12 times.
current (aka Currentmember) is also standard MDX and allows for retrieving the current value of a hierarchy/ dimension.
In icCube I'd add a function to navigate to the previous year so you can reuse it (and fix one if needed). Like :
WITH
FUNCTION timePrevYear(t_) AS ParallelPeriod( [Time].[Year], 1, t_ )
MEMBER [Sales Ratio] AS DivN(
[Sales],
( timePrevYear( [Time].current ), [Sales] )
SELECT
[Time].[Jan 2015]:[Time].[Dec 2015] on 0,
{[Sales],[Sales Ratio]} on 1
FROM
[Cube]
It's going to be a bit too much but eventually you could add this kind of calculations in what we call in MDX Utility or Stats dimension, so you can even let the end-user select this in a dropdown from a reporting tools. More on this here.