What do you do with a foot?

I first heard the expression 'step foot' 20 or 30 years ago, when I dismissed it as a one-off mistake; or maybe, as I was working at the time in  a US-based company, I imagined it was a feature of American English (AE). But I've been hearing it more and more, and have finally looked at a Google Ngram that compares the two.

The expression 'step foot' made a negligible impression until the early 1990s

<autobiographical-note>
I'm reading at the moment a novel set in the time of Henry (Tom Jones etc.)
Fielding: 

The 'guilty' novel

The 'guilty' novel

It would be ridiculous to expect a modern novelist to write all the dialogue in a historical novel cast in the vernacular of the day; without linguistic anachronisms it would simply be unintelligible. But I don't think I have ever seen this phrase in print, and I had only just seen this Google Ngram (or maybe, now I think of it, the fact of my interest in the expression made me notice somethng I'd been seeing for years but not paying attention to).

Anyway, for whatever reason, I did notice when an eighteenth-century character used a late-twentieth century speech form.
</autobiographical-note>

But that Google Ngram lumps together by default all  English appearances of the queried text, from Google Books in general. It is possible though to narrow it down to AE or BE.

<tangent>
...but not Australian English or South African English, or....Besides, what does 'AE' mean? Does it include Canadian English, or ia 'A' just an abbreviation for 'USA'? And besides,  how can a Google Books title refer to any particular geography? Does 'BE' include Indian English and all those other Englishes that publsher's rights contracts refer to as relating to the 'traditional British Commonwealth' (or did when I last had to deal with such things, in the early 1980s)?

So many questions, so little time...
<tangent>

And AE, as usual, takes the lead in the evolition of English:

The 'step foot' line begins to heave itself up off the x-axis as early as the 1980s. By contrast, in BE that line shows no significant sign of life until the mid-late noughties, when in AE it had been growing increasingly strong for 30-odd years And whereas in AE 'set foot' outnumbers 'step foot' about 10:1 in BE the number is more like 15:1; give it a few years though. 

(I'll never use the arriviste 'step foot', but my grandchildren may. Speaking of which, as I'll be seeing them all tomorrow, there are things I should be getting ready....)

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