Updated.

What happens if

LabelSetting referenceLabel = LabelSetting.valueOf(preferenceStore.getString(PreferenceConstants.P_PCR_REFERENCE_LABEL));

the return of preferenceStore.getString(PreferenceConstants.P_PCR_REFERENCE_LABEL) is null or "" or doesn't match any enum name?

	private String getString(Properties p, String name) {
		String value = p != null ? p.getProperty(name) : null;
		if (value == null) {
			return STRING_DEFAULT_DEFAULT;
		}
		return value;
	}

will never return null. STRING_DEFAULT_DEFAULT is an empty string. I added a fallback to the default value instead of no label. Also added enum parsing that will not crash the widget, but also go to a fallback.

I now also set the ID of line series and use the description field instead.

Looks good. Two comments:

A) When introducing a new dependency, I recommend to also set the required version in the MANIFEST.MF:
com.google.guava;bundle-version="27.1.0"

B) I recommend to use as less dependencies as possible. In this case, Google Enums is not necessarily needed. You could rewrite the code in the following way, which is also more compact:

private String getLabel(IWell well) {

	LabelSetting labelSetting = getLabelSetting();
	switch(labelSetting) {
		case SAMPLENAME:
			return well.getSampleId();
		case COORDINATE:
			return well.getPosition().toString();
		case COORDINATE_SAMPLENAME:
		default:
			return well.getPosition().toString() + ": " + well.getSampleId();
	}
}

private LabelSetting getLabelSetting() {

	try {
		return LabelSetting.valueOf(preferenceStore.getString(PreferenceConstants.P_PCR_REFERENCE_LABEL));
	} catch(Exception e) {
		return LabelSetting.COORDINATE_SAMPLENAME;
	}
}

Rebased, removed Google Guave, initialized the setting and put the settings button on the right.