使用与周围行数据的间隙距离成比例的值填充数据中的空白？

At some point soon I'll have to prepare a list of prices of items on days. The granularity is 1 day and on days when there are sales of an item, I'll average the prices to get that day's average. There will be days where no sales are made, and I'm suited that an adequate approximation can be used by pulling the previous and next occurrences of sales, and for each day between them having a price that transitions linearly from one to the other.At some point soon I'll have to prepare a list