Thank you for contacting us.
The control doesn't provide functionality to set the width of each category slot separately. When working with categorical data the chart creates categories for each distinct category value in the source collection of the series. Each category slot on the axis has an equal width with the other slots. So, for example, if the whole plot area have size 200 pixels and there are 2 categories in the data, the chart will create two categorical slots with 100 pixels width each. If there are 5 categories each slot will be 40 pixels wide.
As a side note, you mark the product of this thread to RadChart which is our old charting component. From your question, we are assuming that you are talking for the new RadChartView so we changing the product of this ticket to RadChartView for WPF.