Allow each bar in a bar chart to have a different width

### Feature Proposal

Currently, the `barThickness` option only takes a single number as input. If we could pass in an array, then each bar could have its own width.

This is similar to properties like `backgroundColor` and `borderColor`, which can take a single value or an array of values for each point in the dataset.

### Feature Use Case

I'd like to make a bar chart where the width of the bar indicates the "size" of that category (the y-axis measures an average of all points in that category).