CSMA/CA and Delay?

If I would draw a graph and on the x-axis there is number of nodes, and on the y-axis the delay, what would the graph look like? Is it a linear graph or exponential?

Here are two papers that describe it is linear, but I'm unsure

But here is the frame delay is having a non-linear function:
https://www.hindawi.com/journals/misy/2016/8967281/fig8/

while backoff-delay is linear:
https://www.hindawi.com/journals/misy/2016/8967281/fig9/