Sorry for not getting back sooner, but with A16's deadline hurtling towards us I've been a bit occupied.
Anyway, I see what's going on here...
Firstly v1.02 basically assumes the entire data file is relevant, i.e. it gets completely refreshed and old data is deleted. Currently to make this read from this sort of "log file" that you're working with... well the problem is that row 0 is the first row, row 1 is the second... etc etc and since you don't know what the last number is you can't explicitly access it. Essentially the data file is a CSV file and the data items point to a row & column.
That said, I've managed to find the time to add a small feature to DataViewer v1.03 which will be bundled with A16 that will solve this, allowing you to specify the number of new rows each update adds to the log file, in other words it can ignore all the old lines and just pick up the new data.
Attached is a working layout that requires DataViewer 1.03 in order to work. If you open it with 1.02 (i.e. with A15 still running) it won't be able to pick up the end data and will still show the first item in the log file, but that's ok, when A16 is out that will fix itself. In the mean time you can still play around with the layout and rename the items in the map file.
There are two layouts in here, one uses the documented method of numerical references, the other a new method currently "in-progress" that makes layout building easier by assigning names to data instead of the index-based approach I (stupidly) implemented in 1.00.
Either way works, both are fully supported going forwards, but the new one is likely to be easier to work with going forwards even though I've not got round to documenting it yet.