A number of Excel features added to the product by Microsoft over the years were originally made by Excel developers. The addition of sparklines added a whole new way to visualize data. Before they were an Excel feature, sparklines were popularized by Edward Tufte. See:
The inspiration for the implementation of sparklines in Excel actually came from Excel developer Rob van Gelder. In this article
he shares the VBA code for the LineChart user-defined function, which places a chart in the cell where the function is called from. You can view the use of this UDF and the VBA code in this example file.
If you are not familiar with Excel’s built-in sparkline feature, you can read the following Microsoft article.