Last week I created a new droplet with the default Ubuntu 16.04.3 x64 installation. Since then, the Memory Used graph shown on the web site has been showing almost twice the amount of used memory compared to what I see on the server in ssh when running "free -h" or "htop".
Right now the graph says 59% used, while running "free -h" shows 144M used of 488M, i.e. 30%. Running "htop" shows 170M/488M used, i.e. 35%, which is still far from 59%.
I only have nginx and two Nodejs instances installed and running, so the server is pretty barebone. I don't expect perfect consistency, but would expect something in the range of 30-40%, not 60%.
It seems like the graph on your site might be classifying the memory buffer cache as used memory. To me, this is incorrect, as cached memory is freed up automatically when used by applications, so it's not a sign that you're nearing your memory limit, which the graph currently suggests.
Because memory management is the most important part of a low-memory server I would expect the graph to report actual memory used by my apps and ignore cache, to give me a much more useful metric.
Here's the complete output of "free -h":
total used free shared buff/cache available
Mem: 488M 144M 15M 5.6M 328M 304M
Swap: 0B 0B 0B