! Removes the specified \a layer and returns true on success. All layerables (e.g. plottables and items) on the removed layer will be moved to the layer below \a layer. If \a layer is the bottom layer, the layerables are moved to the layer above. In both cases, the total rendering order of all layerables in the QCustomPlot is preserved. If \a layer is the current layer (\ref setCur
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