[Bug][Jupyter Widget] Legend color schema changes with respect to data

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. load kepler
2. Add data & config
3. Export map
As you can the legend colors keep changing as the legends are creating with respect to the data.
so if the initial legend of snow is green, it is no more green as it changes with respect to data which is very confusing

**Expected behavior**
legends should be generated with respect to unique values in a column instead of chosing a specific data range. or at least give a choice to choose as an arg whether to generate legends from entire column or the selected data of the column itself

**Screenshots**
Also I explained the issue in the slack channel screenshot below

**Environment (please complete the following information):**
- Python version: using python 3.6
- keplergl Widget version 0.1.2

**Additional context**