Plot group color is not binded to the group value

#### Wave SDK Version, OS

wave 0.12.1, Linux

#### Actual behavior

When using ui.plot([... color='=product', ...]), a different color is assigned to different values of the "product" column of the data. However, the same product value can get a different color assignment (probably it depends on the sequence in which the values are in the data). So there is no way to map a "product" value to a color.

#### Expected behavior

If it works as designed, then we should move it from bugs to improvements.