How did scientists figure out the expansion is accelerating?

[resurgance2001]

I understand that the acceleration of the universe's expansion was discovered by looking at very far away Type 1a supernovae. My question is how was the data used exactly to calculate the Hubble constant in the past and then compare it with today's value? Did they simple plot the distances against recession velocities in the normal way but obtain a graph that was not a straight line? Thanks

 

[PeterDonis]

Did they simple plot the distances against recession velocities in the normal way but obtain a graph that was not a straight line?

More precisely, they plotted distances derived from two indirect measurements, angular size and brightness, against recession velocities and obtained a graph that was not a straight line. That in itself was not surprising; the graph would not be expected to be a straight line. But the particular shape of the curve indicated decelerating expansion until a few billion years ago, then accelerating expansion--in other words, it had an extra inflection point in it that wasn't expected.

A brief description is here:

https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe#Evidence_for_acceleration

Note that other pieces of evidence for accelerating expansion are also given.