I assume you are grouping on something like just the numerical month (i.e. 11, 10, 9 ... 12, 11) in the chart? If there is nothing to make it distinct from the other value it will be grouped together. You need to add another level like Year in.

You should try to make you query something like below:

SELECT

Year(Times) AS [Year],

Month(Times) AS [Month],

SUM(NumberOfCars) AS TotalCars

FROM

MyCars

WHERE (Times > DATEADD(year, - 1, GETDATE()))

GROUP BY Year(Times), Month(Times)

Then in SSRS chart Category Groups have Year AND Month in there. This way it'll know November 2012 is distinct from November 2013.