Wordpress depending on the plugins that you are running can require quite a lot of resources and the problems will start when concurrent connections increase. There is usually a resource that becomes pegged at that point, which is most likely CPU, and potentially RAM.
Based on your traffic graph that you provided it looks like 95% of the time the traffic is very low and then suddenly you get a large spike.
What you can do is use a load test to simulate the number of active concurrent connections and then see how your resource usage changes as those connections are processed.
So if you are normally receiving 1-10 connections per second look at your load. Then increase it to 10-25 and look at how load across CPU and RAM changes, then increase it to 25-50, 50-100, 100-200, etc.
That will show you the corresponding load on the system and eventually you will get to a point where the system is overloaded and the website stops loading, which will indicate, ok, this is the breaking point.
But now you will have an idea that a Droplet the size of X, runs out of CPU/RAM at Y load, so you can either increase it if you anticipate a future spike, or work in some more basic caching. Supercache is good, but it's still a wordpress plugin, which means that it will still take up a lot of resources.
What we've done in the past is actually setup two separate web servers on the same system. One that is actually processing wordpress requests and a separate lightweight nginx server in front of it, that is reading off of a very basic static cache which is regenerated every 5-10 minutes. Usually the system is overloaded with requests to your index, or to a particular blog post, so this simple static cache handles those requests at 99% less load because it isn't doing anything dynamic. It's not an ideal solution, but it gives you some breathing room to figure out something more long term.