I'm interested in your source for that graph.

Googling a bit for stuff by Sommerville, I come across a pie chart for "distribution of maintenance effort" which has all the hallmarks of a software engineering meme: old study, derived from a survey (such self-reports are often unreliable owing to selection bias and measurement bias), but still held to be current and generally applicable and cited in many books even though more recent research casts doubt on it.

Here's a neat quote from the linked paper (LST is the old study):

(Possibly) participants in the survey from which LST was derived simply did not have adequate data to respond to the survey. The participating software maintenance managers were asked whether their response to each question was based on reasonably accurate data, minimal data, or no data. In the case of the LST question, 49.3% stated that their answer was based on reasonably accurate data, 37.7% on minimal data, and 8.7% on no data. In fact, we seriously question whether any respondents had ‘‘reasonably accurate data’’ regarding the percentage of effort devoted to the categories of maintenance included in the survey, and most of them may not have had even ‘‘minimal data.’’

I love it that 10% of managers can provide a survey response based on "no data". :)