What is this kind of graph called? How do I interpret it? What is it useful for? I can't even tell what the axes mean.
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I'm not sure what the chart is called, but here is how to interpret it:
The x axis is price, and the y axis shows cumulative buy and sell orders on (probably) a bitcoin exchange. Where the different colours meet in the middle (with a small gap, generally) is essentially the current market price.
The z axis is how it changes over time.
The screenshot shows the price dropping from just over $90 at 1:00 to $88 at 2:00.
The green cliff shape at $86 is a "buy-wall" (ie a substantial amount of buy orders) that doesn't seem to have substantially changed over the period shown.
It might be useful to someone trying to make money by trading between the currencies (in this case, almost certainly bitcoin and US dollars), or perhaps just someone trying to pick a time to perform a one-off buy or sell.
It would make sense that the style of chart would be applicable to trading of any currency (not just bitcoin/USD), securities, and commodities etc.