This a poor graph to use to visualize the mempool in the context of working out a fee. A better choice is the third graph on Johoe's site here (https://jochen-hoenicke.de/queue/#1,8h) entitled "Mempool Size in MB".

The reason for this is because the graph you linked to shows the number of unconfirmed transactions, whereas the graph I've linked to shows the size in MB of these unconfirmed transactions. Why is this important? I'll use two examples to illustrate my point. Lets say first of all that the mempool contains 10,000 transactions, but each transaction is less than 250 vbytes in size. This means the mempool is actually relatively empty at only 2.5 MB, and could completely empty in only 3 blocks. Conversely, lets now say the mempool contains only 1,000 transactions, but they are all huge consolidation transaction from various exchanges, and each one is 30,000 vbytes in size. Now the mempool is the fullest it has been in months at over 30 MB.

So using the graph you linked, if we see 1,000 or 10,000 transaction, we don't actually know how full the mempool is because we don't know the size of those transactions, which is what actually matters. Blocks are limited by size, not by number of transactions. A block could contain anywhere from 0 to over 4000 transactions.