Using GLE 4.2.4c on Ubuntu 14.04
The attached example (example.gle with data file example.dat) defines two data sets, one of them containing '*' values, i.e. non-existing values. If the difference of these data sets is plotted, the entry with the non-existing value is not plotted - which is correct.
However, if the data sets are modified by a let-expression and then the modified data sets are subtracted, the plot shows a (interpolated?) value for the non-existing one - which is not what I would have expected.