Display a warning when some combinations are not represented in the plot

[lldelisle]

Hi,
Thank you very much for the tool. It is really nice. However, I think it is better to be warned when a combination is not represented.
Lucille

[coveralls]

Coverage decreased (-0.8%) to 78.666% when pulling 65d2cd8 on lldelisle:master into 032e784 on jolars:master.

[jolars]

Thank you for your contribution. I disagree, however. Plotting diagrams with some missing combinations is really just a feature of the program and will be unavoidable for large combinations. I don't plan on having warnings thrown every time it happens. I have plans to provide a separate plot for diagnosing the fit.

Also not that you should use warning() and not message() for warnings.

Thanks again.