modifying categorical data causes confusion

[boygirl]

From @boygirl on June 22, 2016 20:0

Currently categorical data is handled by creating a string map, and replacing string x values with mapped numeric values and storing the associated strings as xName. This becomes confusing when users are trying to work with functional styles and props. Rather than modifying x we should be creating a new property on the data objects that will be used always. Maybe victoryX and victoryY

Copied from original issue: FormidableLabs/victory-chart#257