I don't know what pluggable actually represents in programming, but it stands to reason that it should be spelt with double g. In fact, it is a monosyllabic verb just like "run", "stop", "cut" and so many others, ending in a consonant which needs to be doubled to preserve the correct pronunciation of the verb when used in the gerund or the past participle (obviously, if the verb is not irregular), for example. The suffix "-able" is the same as the suffixes "-ing" or "-ed" would be.
NOTE. As for the presence in the NGram of "plugable", I agree with David Wallace's comment above, that is that someone misspelt the term once in writing and others followed in the mistake. This adds to my mistrust towards the usage of NGrams versus reasoning.