Is there somewhere a statistic chart showing this for the last months?
If you want to see the history of the required transaction fee, I think the best option is
jochen-hoenicke website.
Set the period at top of the page and move the mouse pointer to see the mempool condition in different times.
The tool displays size of unconfirmed transactions in the mempool at different fee rates.
Let me explain how the tool works with an example.
Look at the image below. (Note that this is just an example and doesn't show the current condition of the network.)
According to the data shown in the above image:
There are 17.7 vMB of transactions paying at least 1 sat/vbyte.
There are 15.6 vMB of transactions paying at least 2 sat/vbyte.
There are 15.0 vMB of transactions paying at least 3 sat/vbyte.
There are 13.9 vMB of transactions paying at least 4 sat/vbyte.
........
........
........
Any block can include up to 1 vMB of transactions.
So, assuming the size of unconfirmed transactions in the mempool is as shown in the above image, the required fee for getting confirmation in the next block is 15-17 sat/vbyte.
Of course, the network isn't that congested now and as mentioned by LoyceV in the above post, 1 sat/vbyte should be enough now.
Note that the required fee is dynamic and can change over time.
There is no way to guarantee that a transaction will be confirmed in the next block.