I have a spectrogram using QCPColorMap and with QCPColorScale, which work beautifully.

The scale is in dBFS, which is always max. 0. However, I would like to put a marker (e.g. a triangle arrow head, but a simple tick would do) on the left side of the color scale, to indicate where the actual max. level of the signal is exactly, i.e. how much below 0 dBFS the max. spectral level lies.

How could I achieve this?