What is minimum safe amount of fee that can be set manually in Electrum wallet?

[o_e_l_e_o]

Scroll down to point 5 on this post (https://bitcointalk.org/index.php?topic=5182906.0) and have a read for a good explanation of how to read the graph you've linked. Also read the text at the bottom of the page the graph is on.

Each colored band represents a fee rate in sats/vbyte.
The size of each colored band represents the total size in megabytes of all the unconfirmed transactions which are paying that rate.
The graph slowly increases over time as more transactions are broadcast and added to the mempool. Whenever a block is found, there is a sudden decrease in the graph as a chunk of transactions are confirmed and therefore removed from the mempool.

Blocks usually take from 1,2 MB to 1,6 MB of space. So, according to this image, if you sent a transaction paying 4 sat/byte or more, it would very likely confirm in the next block.

This graph shows the virtual size, not the true size, so blocks will take a maximum of 1 MB. This is explained at the bottom of the page:

So for the BTC and LTC chains, a block will always take at most 1 MB from the mempool, even if it is bigger than 1 MB, because the lower diagram already shows the size in vbyte (with the segwit discount included).

[WLtoys V912]

Thanks for your explanations.
This concept seems more complicated than I expected but I will tell what I have understood.

According to what you said and what I see in the mempool graph (I mean the picture I uploaded myself), the biggest colored band is BLUE. It means most unconfirmed transactions are related to those who have chosen between 1 and 8 sat/byte for their transactions. So, if I choose this range of fees, my transaction takes longer to be confirmed by a miner.
In addition to this, the YELLOW color seems to have a small band which means it includes a small number of unconfirmed transactions (compared with BLUE band). So, if I choose anything between 80 and 140 sat/byte I have a much bigger chance to receive confirmation faster.

This is what I understand from the graph. Correct me if I'm wrong.

[o_e_l_e_o]

This is what I understand from the graph. Correct me if I'm wrong.

This is correct. You can get a more accurate feel for things if you mouse over the very right hand side of the graph, as shown in the picture that BitCryptex shared.

As you can see from that picture, when you mouse over it tells you the sum of all the values above each band. So although there are 37.839 MB of transactions in the mempool, only 1.859 MB of them pay more than 2 sats/vbyte.

If you want to be confirmed within the next block, then I usually aim to pay within 0.1 MB of the tip of the mempool. In BitCryptex's picture, that would be 170 sats/vbyte. However, a fee of only 80 sats/vbyte would put you within 1 MB, and 2 sats vbyte would put you with 2 MB, which are both fairly good bets for getting confirmed soon.

Using this graph also lets you see the recent trend of the mempool. If it is rapidly filling up, then you'll need to pay higher than you otherwise would for a rapid confirmation. Conversely, if you see it steadily emptying, then you might be able to pay lower and still be confirmed relatively quickly.