Reverse engineering SARS' queueing system

On Monday I had to visit SARS to submit an IRP6.  I'm what's called a "provisional taxpayer", which means I get to deal with the taxman about twice as often as people who aren't.

Previously their queueing system was just a long line of people waiting their turn.  Then, some years ago they started offering chairs - a little more comfortable, but the queueing system was basically the same: one's seat determined one's position in the queue, so it was like a big game of Musical Chairs.  And finally, about a year ago, SARS started issuing numbers at the reception desk: one could now sit anywhere and simply wait right there until one's number came up.  The numbers appear on a screen, but also, a Stephanie Hawking type computer voice reads the numbers aloud.

I had been trying to read a few pages of The Great Disruption, but Stephanie's staccato voice proved too distracting - like a numbers station's siren call.  So I decided to make lemonade.  For a bit more than an hour I jotted down the time and the new number - with the aim to write exactly this post.

Now that I see the graph, it's a bit disappointing.  I was hoping to see points along more lines of different slopes and intercepts than the two segments visible here - which I know both to be from the "INCOME TAX RETURNS" stream.  (By the way, why are these things always in all-caps?)  There seemed to be multiple virtual queues running simultaneously, each allocating numbers from distinct ranges.  Perhaps I just didn't collect enough data - perhaps somebody needs to spend a whole day there.  Maybe next time.