Roughly half of its staff work in research and development, spending on which grew by 26% last year, to $3.4bn—more than the company generated in net profit.

●原句分析：which 是关系代词，指代先行词 research and development（研发）。句中的 spending 是名词，意思是“开支，支出，开销”。on which 在此为后置定语修饰名词 spending，spending on which 在含义上相当于 spending on research and development（在研发上的支出）。

其实说成  on  which spending 也不是不可以，下面举H. W. Fowler 的 A Dictionary of ENGLISH USAGE 中的一个类似例子：

“Besought”is the established past & p.p., though “beseeched”, on which the OED comment is merely “now regarded as incorrect”, still occurs, probably by inadvertence, & Milton has “beseecht”. 

句意：Besought 是（beseech）公认的过去式和过去分词形式。尽管 “beseeched” 仍有使用——《牛津英语词典》（OED）对其的评论仅是 “如今被视为不正确”——但这种用法可能是出于疏忽，此外，弥尔顿还用过 “beseecht” 这一形式。

分析：句中的on which the OED comment 也可说成 the OED comment on which（which指代beseeched），意思是“《牛津英语词典》对 beseeched 的评论”。

●原句大意：该公司大约有一半员工从事研发工作，去年研发支出增长 26%，达到到了34亿美元——这一数字超过了公司的净利润。

●原句改写：Roughly half of its staff work in research and development, and spending on research and development grew by 26% last year, to $3.4bn—more than the company generated in net profit.

●类似例子：下面的截图来自H. W. Fowler的 A Dictionary of ENGLISH USAGE：

The only satisfactory rule, exceptions to which are very few, is this...

句意：唯一令人满意的规则（其例外情况极为罕见）是这样的……

分析：which 也是关系代词，指代前文的名词 rule。to which 在此作后置定语修饰名词 exceptions（所以 to which 被置于 exceptions 后面），exceptions to which 在意义上相当于exceptions to the rule（这条规则的例外情况）。