BUG: when adding multiple plots with different cmap, colorbars legends use first cmap
For example, when doing twice a scatter plot with a different colormap, it adds a colorbar for each, but with the wrong colors:

```
df = pd.DataFrame({'x': [1, 2, 3], 'y':  [1, 3, 2], 'c': [1, 2, 3]}
df['x2'] = df['x'] + 1   

import matplotlib.pyplot as plt 

fig, ax = plt.subplots()  
df.plot('x', 'y', c='c', kind='scatter', cmap="cividis", ax=ax)
df.plot('x2', 'y', c='c', kind='scatter', cmap="magma", ax=ax)   
```

![Figure_1_2](https://user-images.githubusercontent.com/1020496/78757174-59ef9300-797c-11ea-8d2b-a305582efb4b.png)
