20051201: IDV - IDV1.3A3: simple difference a - b problem for charts

Hi Steve,

> I tried to produce a simple difference a - b computation for a time series chart and received the error above. I referred to page 53/190 of the IDV users manual for guidance. Both "a" and "b" are in the same RAF aircraft file. Can "a" and "b" be in the same data source? Is that the "duplicates are not permitted" problem? 
>
>  
The problem you are having is in how we internally structure the track 
data as a visad data object.
What's happening is we are subtracting the field value *and* the times. 
Of course the times are the same
so we end up with 0 - thus the error with the duplicate times.

There is no quick fix to this but we will put it on our short list.

-Jeff