Running sum in webi 4.1

hi guys

i need help to create line graph and for that i need to calculation:-

runningsum(salary - bonus)

after running sum, need to take out the minimum value of running sum and if it is negative value then make it positive... with abs(min(runningsum))

after getting minimum value, needs to add minimum value to running sum value.

could you please help me in to create min+runningsum column......how to do that... i am able to do till min(runningsum)

Using Webi 4.1

sample data is like this :-

Date           salary-bonus      runningsum      abs(min(runningsum))           min+runningsum
2015-01-01      5                     5                          6                                         11
2015-01-02      -2                    3                          6                                           9
2015-01-03 -     1                     2                         6                                           8
2015-01-04      8                     10                        6                                         16
2015-01-05      -7                     3                         6                                           9
2015-01-06      -9                     -6                        6                                           0