This page displays the number, fee, or weight of the unconfirmed transactions, also known as the transactions in the mempool (the transactions that haven’t been written to the block-chain yet and kept in the volatile memory). It gives a real-time view and shows how the mempool evolves over the time. The transactions are colored by the amount of fee they pay per virtual byte. The data is generated from my full node and is updated every minute. Note that in decentralized cryptocurrencies there is no global transaction mempool; every node keeps its own set of unconfirmed transactions that it has seen. The mempool is also cleared when I reboot my node. The idea is based on the retired service bitcoinqueue.com.
The data is separated into different fee levels given in satoshi per bytes. The lowest colored stripe is for transactions that pay the lowest fee. Higher fee transactions are stacked on top of it. Since miners prefer high fee transactions, a new block usually only removes the top-most 1 MB worth of transactons from the queue. If a colored stripe persists over several hours without getting smaller, this means that transactions paying this amount of fee are not confirmed during this time, because there are higher paying transactions that take precedence.
The horizontal axis is time and you can choose the range from the last 2h to all. The vertical axis of the chart can be switched between count, fee, and weight. In the weight chart the height of the graph reflects the total transaction size instead of the number of transactions. If a stripe on the weight chart is much bigger than on the count chart, the transactions in this stripe are larger than the average. Similarly, in the fee chart, the height reflects the total amount of fee the pending transactions pay.
You can click on some fee level in the legend to hide all fee levels below that level. This way you can better see how many transactions are competing with that fee level.
Note that sizes include the segwit discount, i.e., the graphs show virtual byte (weight divided by four). For segwit transactions, the real size of the transaction is a bit larger than the virtual size. 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 weight diagram already shows the size in vbyte (with the segwit discount included). The segwit discount is also included when computing the fee level for a transaction. In case a transaction pays exactly the fee that defines the boundary between stripes, it is included in the higher stripe. Free transactions are not included, even if they make it into the mempool.