def merge_aes(layer1, layer2):
    for key in layer2.aes.keys():
        if layer2.aes[key] is None:
            layer2.aes[key] = layer1.aes[key]
Merges the aesthetics dictionaries for the two layers.
dataset/ETHPy150Open pydata/pandas/pandas/tools/rplot.py/merge_aes