We think that usually the datetime continuous axis would be a better choice for a financial chart. Otherwise, if you have too many categories, you risk of the axis getting cluttered by so many ticks.
Anyway, if you decide to keep the categorical axis, you can still use a similar approach. The categorical axis has a Categories collection that contains all the categories. You can get the relative zoom from the chart's HorizontalZoomRangeStart/End properties and calculate the first and last visible categories. So, you can still get the items in this range. I hope this helps.
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