Just a small QoL update, Premium users can now enable the "Split Graph" button on their item price graphs.
This will display both the buy and sell price of an item on the graph simultaneously so you can get a better idea of the item's activity over a given period, and if the item is spiking.
Below is an item generally has a good margin, you can see that the lines are separated:
If you can only see a red line, then the item usually has a small margin.