Look at the top sawtooth edge of the graphs. As transactions accumulate, the graph climbs. When a block is mined, that block's transactions are removed. This causes the graph to suddenly drop.* You'll notice that usually the sawteeth are uniform - climbing, drop back, climbing, and drop back. Sometimes the jaggy keeps climbing a while before dropping. This happens when a block is late. Other times there are two or more drops in quick succession. This means that blocks were mined (much) faster than 10 minutes apart.
* The software is obviously sampling at some interval detached from the block mining times. Some time after a block is mined, the charting software merely captures smaller numbers to graph. It then draws a diagonal line between the previous (higher) sample and the new (lower) sample. In reality, it should drop straight down.
Look at the bottom of a jaggy. Notice what color is found there. Look up that color in the legend. This tells you that transactions at that fee level were mined in that block. Look at the colors higher in the jaggy. Convert them to fee levels. That means some transactions paid that much to get in the block.
If you are looking to save on fees, look for the lowest bottom of a jaggy within the timescale of when you want the transaction to confirm. Translate that color to the fee. Beware, the graph is of past data. You don't know what's coming in the future. New transactions may flood onto the network. Your cheap fees may run away from you. You may have to wait weeks before fee levels come into the range of your transaction.
If your transaction must be in the next block, choose a color band at the top edge of the jaggies.
If you'd like your transaction in the next block, but are OK with waiting a bit, pick a color band that ends in middle of a sawtooth drop.
The top graph is the number of transaction. This plots the data without giving emphasis to any particular transactions.
The bottom graph is the size of waiting transactions. When (lower) color bands in this graph are bigger than the top graph, you can infer that the cheap transactions are large, involving many inputs and/or outputs. Exchanges or merchants consolidating transactions will be here. This graph is useful to estimate how many blocks it would take to mine everything: Imagine taking 1MB slices off of this graph. How many would it take? Keep in mind that new transactions are always arriving.
The middle graph is the amount of fees pending. This isn't as useful. Sometimes there is a sudden bloom of a higher color band that's totally invisible in the top graph and slightly visible in the bottom graph. This means that someone flooded the network with large moderate fee transactions. This could be an exchange that needed to consolidate UTXOs *NOW*. The reason for the bloom is that this is a nonlinear transformation of the bottom graph - the higher fee levels get magnified.
Sometimes you'll notice the bottom band reducing in size. It is possible that a miner mined their friend's cheap transactions. However, in almost all cases this instead means that the mempool was full and this node discarded the cheapest transactions. You'll usually notice the bottom band reduce as a higher band increases.