text: Title: Add alpha support for scatter plots
  Body: **Describe the solution you'd like**
  When I'm plotting scatter plots (such as PCA), oftentimes there is a bunch of data. By using the ``alpha`` parameter on matplotlib (also available through the pandas plotting interface), it allows you to discern the data and amount of overlapping.
  
  **Is your feature request related to a problem? Please describe.**
  In general, I think an ``alpha`` parameter to any scatter plot visualizations (``PCADecomposition``) would be awesome. It would also be inline with what pandas plotting provides.
  
  **Examples**
  Seaborn scatter plots come with a slight alpha applied. But you can use ``scatter_kws={'alpha':0.3}`` to set it. In pandas or matplotlib you just pass ``alpha=0.3``.
  
  Jake's book also has alpha turned on when plotting scatter plots. See https://jakevdp.github.io/PythonDataScienceHandbook/05.09-principal-component-analysis.html
  