Add custom colors to linear scale type charts

[Yuqing Liu]

Is your feature request related to a problem? Please describe.
Currently, custom color is only available to ordinal-scaled type charts. It is not available for heat map, linear-scaled bar charts, vertical charts, etc.

Describe the solution you'd like
Add custom colors for linear scale type charts in the form of gradient color stops.

Describe alternatives you've considered
n/a

Additional context
n/a