I'm currently working on a plugin for excel (using VB.net--not VBA) that will attempt (among other things) to recognize the format of the source data of a chart and configure the chart's series' number format strings appropriately. That is, yearly data should always be two numbers, monthly data should always be a 3-letter month followed by a 2-digit year, etc. The goal is to have a uniform representation across all charts for monthly data, annual data, daily data, etc.

My question is this: how do I find the number format string for the cells (or any cell that is part of that series, as I will be assuming all cells that make up a particular series have the same format) that represent the source data for the chart?

Note: I can assume that the chart will always point to a range of cells (not, for example, an array of data).