SQL & PL/SQL :: Perform SUM On Two Columns Based On Level?

Below is the code that i tried to perform the sum operation.

CREATE TABLE TEST11
(
FISCAL_TIME_ID NUMBER,
data_id number,
M_VALUE NUMBER,
Y_VALUE NUMBER
);

[code].....

The result I got is

SQL> SELECT FISCAL_TIME_ID, DATA_ID, M_VALUE,
2 SUM(m_value) OVER (PARTITION BY fiscal_time_id, data_id
3 ORDER BY FISCAL_TIME_ID) AS YTD_VALUE
4 from test11;

[code].....

But what I am actually want to get is.

FISCAL_TIME_ID DATA_ID M_VALUE YTD_VALUE
-------------- ---------- ---------- ----------
20110500 3 2 2
20110700 3 50 52
20110800 3 52
20111000 3 250 352
20111100 3 300 652

That is, the YTD_Value column is nothing but sum of M_VALUE column + previous fiscal_month_id's ytd_value column.

Test case:

The YTD_VALUE for fiscal_time_id 20110700 is obtained as current M_VALUE + previous fiscal_time_id's ytd_value => 50 + 2

I tried with the SQL but i could not get the result.