I need to create bollinger bands in Numbers

Question

I am trying to chart a performance margin on a chart.  I tried to chart a shaded area around a moving average using plus an minus STDEV's but the problem is that the STDEV cant show a negative number and this moving average travels into negative territory.  Below is what i have. 
for the moving average:  =SUM(X815:X838)/25
For the top and bottom areas for the shaded area: =AC839+STDEV(AC815:AC838)
                                                                                         =AC839-STDEV(AC815:AC838)

Answer 1

Below is two hundred values from one of my data sets(there is more than 800 so far) .  These numbers are represent a ratio of the profits or losses in relation to the risk taken in a financial position. 
The initial picture that i posted showed this data as bars.  The 25 period average of these numbers in a red line [e.g. =SUM(X13:X36)/25]  the shaded area in blue is the MA+25 period STDEV and the white-out portion is the MA- 25 period STDEV
P/L%
9.30
3.59
1.55
-0.857
-1.26
-1.04
-1
-1.07
1.80
-1.05
2.50
-0.937
-0.38
0.176
-0.0476
-2.4
-1
2.87
3.72
-2.5
-5.00
2.54
-1
-1.19
-0.115
1.64
0.167
-2.69
-0.375
36.2
0.444
-0.00000000000
-0.406
-1.33
-0.771
9.54
0.148
-1.05
2.89
-0.800
0.735
-1.88
-0.667
2.3
-2.38
-1.1
-1.29
-0.105
-0.0235
-1.83
-0.333
-3.25
-0.667
-1.06
-2.10
1.23
-0.964
0.100
-0.898
10.6
-0.480
0.696
-2
-0.138
13.6
-0.812
-0.727
0.789
-1
4.57
-0.477
-0.778
-1.78
-0.189
-1.29
3.85
-2
-1.54
-1.53
-1.09
-1.3
-0.78
-0.722
-2.20
0.400
-2.25
0.0852
-0.286
0.883
-2.75
5.17
0.286
-1.20
-0.389
-1.96
-1.33
-4.31
8.03
-0.0262
-3.00
-2.33
-0.842
1.44
0.250
-1
-1.11
-1.69
-0.889
-0.685
-0.800
-0.778
-3.17
-7
1.02
-1.10
-1
-2.00
-1.20
1.8
-1.25
-0.667
-2.08
-0.944
0.385
1.78
-1.60
-1
-3
0.645
-2.42
-0.00353
-1.24
-0.400
-1.21
0.551
-1.25
0.333
4
-0.227
0.0769
1.93
-0.135
-1.80
2.85
4.80
-3.50
0
1.92
-1.57
1.50
3.00
6.82
0.725
-0.566
-1
-1
-1
2.52
-0.800
-0.444
2.58
-0.333
-0.154
2.3
1.75
0.47
1
-1.6
-2
-0.553
-0.778
-0.923
-0.429
-1.03
-0.383
-0.923
-3.20
-2.4
-6.50
-0.778
-3.25
-2.22
-0.667
-0.0625
1.67
-0.333
-3.50
-0.519
-2.93
-5.08
-2.70
-2.80
-1.67
1.16
-0.941
-0.136
-1.66
5.67
-0.286
-1.05