The simple indicator which shows the candles of the middle price of the bid and ask (open = (bid open + ask open) / 2, high = (bid high + ask high) / 2 and so on).
The indicator also shows the ask high and bid low dots as well as the most recent price. You can apply other indicators as on the bid/ask prices (whichever is chosen in the chart toolbar) or to these midprice candles (see the "Source" tab in the indicator options).
The indicator also shows the current bid and ask prices as a lines.
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