As @ballardw said, you will not be able to do this directly using VLINE. You will need to calculate the standard deviations externally and flag the values that drop below zero. Then, combine a SERIES and a HIGHTLOW plot to draw the graph. I would recommend using a HIGHLOW plot to represent the STDDEV values, as you can put different styles of endcaps on the lines from the data set.
In the data step to process your +/- STDDEV value, create a column called LOWCAP (or whatever you want to call it) to store the symbol to use on the low end of the HIGHLOW lines. If the low value is >= 0, set the value in this column to be "SERIF". If the value is < 0, set the low value to be 0, and set the LOWCAP value to be OPENARROW. Then, set the LOWCAP option on the HIGHLOW plot to point to this column, while setting "SERIF" on the HIGHCAP option.
Hope this helps!
Dan
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