It is liner. I have checked it in my case. RC is simply a function of SP and hence as you grow your SP, your RC will also grow and its grows linearly. But you must note one important point, that even if someone has zero SP, some base value RC still remains so that a user can at least interact a little, may be one transfer or may be one upvote. And from that base value it grows linearly.
You can refer to this.
Here you can see it has zero SP yet it has some RC and you can also note that the adjustment is being shown as 3.022 SP and this is that amount which is burned at the time creating a steem account and that is why a base value RC is allocated to a newly created account and from there the RCs grows linearly in proportion to the SP.
I think that it is more close linear distribution. Because the amount of RC (resource credits we can have are similar to the amount of our Steem Power.
I currently (2019.03.15, 11:00 CET) have 295.376 Steem Power, and the maximum RC (resource credits) I can have at the moment is 296M. The adjustment is 1.010 Steem Power.