vd id -0.4 1.84426548556218E-21 -0.2 1.04706618384119E-19 0 5.94463E-18 0.2 3.37501357433389E-16 0.4 1.91613550833913E-14 0.6 1.08786978346975E-12 0.8 6.17628899749531E-11 1.0 3.50653601747389E-09 1.2 1.99080626680972E-07 1.4 1.13026347718055E-05 1.6 0.000641697562011 1.8 0.036431838186809 2.0 2.06838690412047
The column Vd are the voltages, Id is the diode current according to the real formula, Id0 is the current with the simplified formula where the "minus 1" is changed to "minus zero". As Id0 is a true exponential curve, you can take the logaritm in column Id0_log. (You cannot take the log of a curve that becomes zero and negative like Id) The plot is from Id0_Log versus Vd. In this plot I made the lowest part dotted, because there it is NOT the actual diode current anymore, but does show the value of Is at the intersection with the Y-axis.
Following the exponential curve to the left brings you asymptotally to zero. But the "minus 1" subtracts an amount of Is, so that the real diode curve goes through the origin and, with negative voltages, shows a reverse leakage current of amount Is.
If the original manufacturers curve would have been on a really large log plot, we could have simply used a ruler to extend the straight line downwards to easily find Is at Vd=0 and then compute N, instead of compute first N then Is with the above macro's. The ruler method has been described in "The Spice Book" by Andrei Vladimirescu (1994).