fix the figure example

[d-morrison]

https://ucd-serg.github.io/qwt/chapter2.html

[github-actions]

The GitHub issue titled "fix the figure example" references the webpage https://ucd-serg.github.io/qwt/chapter2.html. It likely relates to a problem or inaccuracy in the figure examples provided in Chapter 2 of the linked resource, which is presumably part of the "qwt" documentation. The issue identifies a need to review and correct the figure example to ensure clarity, correctness, or improved alignment with the intended content. Further clarification or specific details about the exact problem with the figure example are likely required.