Free memory is wasted memory.
I always get a good laugh from the people who spend hundreds of dollars on 32 GB RAM upgrades only to then install those ****** "memory cleaner" apps that keep purging it so that the "free" number is huge.
Nothing like making your RAM useless!
Your graph is normal - OS X uses RAM as much as it can to speed up the system, so you want your amount of free RAM to be very small. You may have a lot of inactive RAM, which is data that is in memory but not being used currently but it has been used recently and might be again. If the system needs it, then it's already in RAM. If you purge this then the "free" number will be bigger, but then if you need the data again the system will have to read it from disk back into memory.
There's no penalty for having inactive memory. If the system needs more RAM it will dump some of that inactive memory as it needs to, but otherwise you're better off leaving it as it is.
The memory pressure graph at the bottom is what you should look at. If it's in the green then the system is having no problem with RAM at all. If it is yellow or red then the system may show performance issues due to a shortage of RAM - this happens if you have a couple of very heavy RAM-using apps open that are demanding large amounts of active memory. If this is a common theme then it's time to look at upgrading. If you're always in the green, you have plenty of RAM.