text: Title: No target coloring in jointplot
  Body: **Describe the bug**
  After assigning `y` values in the `fit()` method of `JointPlot`, no heatmap of such target is drawn on the samples
  
  **To Reproduce**
  
  ```python
  import numpy as np
  from yellowbrick.features.jointplot import JointPlot
  
  X = np.random.rand(100, 2)
  y = np.random.rand(100)
  
  viz = JointPlot(columns=[0, 1])
  
  # here, fit_transform is just fit
  viz.fit(X=X, y=y)
  viz.show()
  ```
  ![image](https://user-images.githubusercontent.com/18686697/97182885-5fd48680-179d-11eb-8db7-e2eebc54b100.png)
  
  **Dataset**
  No. 
  
  **Expected behavior**
  There should be a heatmap to color the samples by the values in `y` as in the PCA case. 
  
  ![image](https://user-images.githubusercontent.com/18686697/97187357-a973a000-17a2-11eb-8628-eee182d39547.png)
  
  
  **Traceback**
  
  ```
  If applicable, add the traceback from the exception.
  ```
  
  **Desktop (please complete the following information):**
   - OS: Debian10
   - Python Version 3.7
   - Yellowbrick Version 1.2
  
  **Additional context**
  