I want to spread this data below (first 12 rows shown here only) by the column 'Year', returning the sum of 'Orders' grouped by 'CountryName'. Then calculate the % change in 'Orders' for each 'CountryName' from 2014 to 2015. this data below (first 12 rows
阅读背景：
R:如何传播，group_by，同时总结和变异。
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